This technically could work. But it would be inpractical. And it would require a lot of work. If you invoke the pad, the keypresses it releases might still be linked to the phone app.

Even if you capture those keys and program a plugin to interpret 222 as 'c' for example and recognize pauses so it would be like a T9 entry system, there is no way to determine if the app is also sending out other types of signals. You can't know unless you know the source and/or monitor every event that happens when you press a number.

It would be much easier to design the whole thing yourself. The problem is with portrait mode. How to force text to be portrait. Making the buttons is simple enough and programming them to respond as they should will not be very difficult. You could create large buttons as a new qwerty layout and write 2/abc, 3/def and whatever. You can write those in portrait mode so the numbers will be legible when holding the phone in portrait mode. The layout is the easiest thing. But even if you do that right, and program the thing right, the text will still be in landscape mode. That is the main problem.