Of course the trick there is... does the keyboard show letters as eliminated? That's the deduction you have to do in mastermind.

And if the keyboard highlighted "right place" letters for the most recent guess it wouldn't be mastermind.

So really it would just show letters you have or have not yet guessed, but without any indication on if they are right, or in the right place.

1. Might be also nice to handle 0-0 case separately and grey out all those letters both in the guesses on the board and perhaps in a separate keyboard as well.

2. Can go one step further, e.g. if a guess has X-0 and all there's only X cells that are not greyed out (see above), mark them as green in all correct guesses on board.

3. After these two steps, step 1 can be applied again since there will be more such cases where it can be applied.

You might argue that it steals from the game, but I'd disagree - it leaves the high level strategy to the player while saving them from dealing with the boilerplate. A very good example of that is Good Sudoku game (app), where they've tried to innovate on classic sudoku interface for similar purposes - I highly recommend checking it out since it's somewhat close to this game in some regards.