I decided to give this a try with a solderless version. Thanks to Pascal for working with me to reconcile some troubles I had with his PinballButtons.apk.
Tips and Caveats:
1. You must install the PinballButtons app from above.
2. You must have the Set Orientation app. Pinball Arcade, for example, must be in Portrait mode.The direction of the Portrait Mode must match what is set in PinballButtons
3. USB Debugging should be turned off.
4. You will need a non-USB On-The-Go adapter. This may depend on whether your tablet supports USB hosting.
Step 1: Mini-Pinball Tablet Cabinet - KBD Controller
Step 2: Mini-Pinball Tablet Cabinet - the Tablets
With the Touchpad, I am using the non-USB On-The-Go cable adapter to connect the Touchpad to the IPAC. This seems the only way I was able to get the Touchpad to recognize the IPAC as a keyboard.
The iPac is plugged into the micro-USB port on the Touchpad and does show up as a keyboard.
With the Asus, I am using a Bluetooth Keyboard Adapter. Pairing is a little tricky. I had to dig up a PS2 keyboard connected to the iPAC passthrough port to enter the pairing key.