Hi! I'm back again with writing guides for Android on these old but lovely webOS devices, because what's greater than having a webOS device? Having a webOS device that can dual-boot Android! (or, even triple boot with LuneOS but that's for another time.) Anyhow, let's get started.

The Touchpad is somewhat tricky on how you can install Android on it, and while TPToolbox makes it very easy, it doesn't support newer versions of Android, and it also doesn't allow an install without Gapps included. So I'll be including a couple of ways to get Android onto your Touchpad.

Also a tiny note, when I say "partitions", I mean "volumes" as the volumes we will be touching here aren't partitions in the traditional sense, rather they are Logical Volumes which do appear like partitions, but work a little different.

1: Easiest Method: Using TPToolbox (doesn't directly support 7.1/9.0 out of box)

To install using TPToolbox, you'll need to setup Novacom beforehand, as it utilizes the webOS Recovery mode to boot the Linux image on your device. If you have recently set up your device following the guide on webosarchive.org, this should already be set up for you. Otherwise, follow the first portions of the guide here.

Next up, let's look into the files you'll need to install Android using TPToolbox.

TPToolbox XDA Thread - For TPToolbox.
Cyanogenmod 11 (Android 4.4) XDA Thread - You WILL need to download this regardless of whatever your target version of Android is. Grab the ROM and TWRP from this thread.
Delta Gapps XDA Thread - Grab the 4.4.4 Gapps from this thread.

Now, let's get into it.

Put your Touchpad into webOS recovery mode. This can be done by holding Power+Volume up buttons at boot.
A USB symbol should appear, make sure that you have Novacom setup and ready to go. Plug in your Touchpad to your PC then launch TPToolbox using the included shell script files.
Now your Touchpad should be booting into TPTouchpad, once that's done, choose "Install Android" by either using the volume button or sliding your finger up and down on the screen, and confirm selection with the home button.
- In the case you'd want to completely wipe webOS from your device, pick Complete Data Reset which will delete all the partitions.
Let it do its thing, then copy CyanogenMod, Gapps and TWRP to the ttinstall when prompted, and safely eject the partition from your PC.
Resize the partitions as you see fit, the media partition is your webOS+visible USB storage. I personally kept mine at 10GB on my 32GB touchpad. If you're installing Android 7.1 afterwards, make sure that the cm-system partition is at least 1400MB or larger. I personally kept mine at 1536MB and left cm-cache at 200MB. cm-data will be your Android's internal storage, Once i've resized media I basically expanded this partition to the maximum allowed.
Once you're finished with resizing everything, TPTouchpad will handle the rest for you and you should automatically boot into TWRP, in which it will also automatically install CM11+Gapps for you.
- In the case the boot fails, simply try to flash again. In the case it still fails, I would recommend backing up your data, then go into TPToolbox and choose Complete Data Reset and then Create webOS volumes, before rebooting your Touchpad back into Recovery mode and then following this guide on webosarchive.org to Doctor your device back into factory state. Once you're done, follow the guide from the beginning.

1.1: Addendum: To Install Newer Android versions.

Download the following:

Evervolv 7.1 (Unofficial with Wi-Fi fixes) XDA Thread - Grab the rom from this thread.
TWRP 3.2.1 - I wasn't able to find the thread that contained this directly, but here's the direct download link.
OpenGapps - This is optional, but if you opt to use Gapps, make sure to pick ARM/7.1/pico for your package.
microG + Aurora Store - In the case you don't want to install Gapps but would still like to retain some Google Services compatibility, this is also a good choice.

Once you got everything downloaded, copy the files to your media storage, the webOS data/USB storage partition, or External Storage if accessing from MTP in Android. Then do the following:

Reboot to TWRP, use Moboot and select TWRP.
Once in TWRP, Flash FLINTMAN-TWRP-3.2.1-0-03-15-2018.zip then reboot to recovery again.
Now that you're in a more up to date version of TWRP, do a factory reset, then flash your ROM (followed by Gapps, if applicable.)
Everything should go well, and you should now be in Android 7.1.2. Note that the first boot will take quite a long time, and every boot after could take minutes to show the Evervolv booting animation.

2: The Manual Method: Installing Android with the help of Tailor.

This method will be way more involved and could potentially risk you a soft brick if you don't symlink everything properly. I recommend you follow the more roundabout method up above instead if you're not comfortable with getting more hands-on. This section will not hold your hand as much since I expect you to know at least the basics of working with Linux.

This method is partially based on the excellent LuneOS install guide for tenderloin. They work more or less similarly in how they can be installed.

I would recommend backing up your data beforehand just in case things go wrong and you need to call the doctor.

Prerequisites:

Tailor (either 0.3.0 or 0.3.1 is fine, I've directly linked 0.3.1 here. Otherwise grab it from Preware.)
moboot 0.3.8 For your bootloader.
TWRP 3.2.1, I've checked and confirmed that it can indeed install Android 4.4, though for older versions, YMMV.
Your Android ROM of choice. Check Method 1/1.1 for 4.4/7.1 downloads.
Means to communicate with webOS' Terminal, Either through Novacom, OpenSSH or Xecutah (on-device, install through Preware.) This also means that Developer Mode on webOS has to be enabled, If you've recently activated your device using devicetool, this is already enabled for you.
A little bit of Linux know-how (Important!)

Once you got everything ready, follow these steps.

Step 1. Partitioning with Tailor:

Install Tailor if you haven't already, once done, reboot your Touchpad and open Tailor.
Unmount USB (media) and Check Filesystem. Then resize it to your desired size.
Tap on Unused Space, and then create Android (system), make sure that it's at least 1536MB if you're installing something like 7.1 or 9.0.
Repeat above, but choose Android (cache) instead, 200MB is plenty.
Once again, repeat for Android (data), I personally use the rest of the space for this.

Now that we have all the partitions set up, lets move on to the next step.

Step 2. Setting up the boot sequence:

Depending on how you want to go about accessing the terminal, copying files will be slightly different. I will assume that you're using Novacom for this sequence.

Open a tty terminal through Novacom, novacom -t open tty://
Remount /boot as Read/Write to modify our boot partition, mount -o remount,rw /boot
cd to /boot, cd /boot
We will now remove the uImage symlink that currently links to uImage-2.6.35-palm-tenderloin, as we will be symlinking moboot to that instead. do rm uImage
Open up another terminal window. On Windows, make sure you're using CMD for this step and not PowerShell, as it can't handle the < correctly in the novacom command. Then do the following:
- Now extract uImage.moboot_0.3.8 from moboot_038-tenderloin.zip that we downloaded earlier, and send that to your Touchpad through Novacom, novacom put file://boot/uImage.moboot < uImage.moboot_0.3.8
- Extract the TWRP uImage (uImage.Recovery) and send it to your Touchpad's boot partition, novacom put file://boot/uImage.TWRP < uImage.Recovery
Now let's symlink everything in place.
- To symlink moboot as a default boot image, do ln -sf uImage.moboot uImage
- Then symlink the webOS uImage, ln -sf uImage-2.6.35-palm-tenderloin uImage.webOS
- Now let's check that everything is symlinked properly, do ls -l, the output should have the following:
  - uImage -> uImage.moboot
  - uImage.webOS -> uImage.2.6.35-palm-tenderloin
  - uImage.TWRP
- If everything above checks out, we can now reboot to moboot. Simply do reboot.
Once rebooted, you will be greeted with the moboot boot picker. Feel free to test everything here. If something doesn't check out, you may need to call the doctor.
Assuming that everything went fine, reboot back to webOS and mount the USB storage, and copy your ROM file alongside any other zips that you might wish to flash like Gapps.
- Whatever you do, do not try to flash Magisk, as it doesn't support the unique uImage arrangement that the Touchpad has.
Now reboot back to TWRP, and do a factory reset. This will only affect the Android partitions which currently doesn't have anything anyway, but it's always good to have a clean slate.
Proceeed to install your ROM, make sure that the Android ROM is the first file in the zip queue, and Gapps being the second if you've opted to install that.
That's it, assuming everything went fine in the install, you will now have Android installed on your Touchpad!

Additional Credits

The old HP Touchpad Index

The new HP Touchpad Index