Oh my mistake, it was actually I who misread the first comment.

Regardless, I don't think a screen tap logger would be necessary in many cases, since you could just open the keyboard on the phone and manually check the bounding boxes for each key.

Making this work on N phones would be more difficult, but not unfeasible. You'd probably just need to know a few things about the phone (resolution, screen size), and it wouldn't be hard to find that information.

Would be especially easy to just target the limited iPhone line.