Please tell me if I understand what it is that you are looking for. You're not actually looking for a solver; you're looking for something that automatically handles pencilmarks.
If that is the case, I have not seen such a program that supports a 15x15 grid. Which is not to say that one does not exist; I have simply not encountered one.
Have you tried contacting the SadMan Sudoku author (
simes) to suggest that he expand his program to handle grids of different sizes?