A good example for what I’m looking for is the custom keyboard used by the WolframAlpha app.
Ideally I would like to set a custom keyboard for each EditText (one that only shows certain characters/digits), although I’m not certain that this is possible.
If anyone can point me in the direction of examples for how to do this and/or provide assistance in any other manner, it would be greatly appreciated.
I have been unable, so far, to find an example using Google of what I need to accomplish.

For developing custom keypad you should check this
android official docs and this sample app
and may this tutorial help you