I have got interested in Dion Cubes
http://www.sudoku.org.uk/images/Dion%20Cube.pdf
and have become so used to using your Sudoku program rather than pen and paper, I have the following suggestions to make:
1: regarding invalid puzzles, which each dion slice is, the ability to dub them - the fact that the program may not be able to uniquely solve a slice and hence provide error propting does not bother me as I always play to be eligible for HoF, so the absence of this would not bother me
2: As I have a lovely big screen I have enough area to tile 9 copies of the sudoku program, and hence use it as a workspace for a dion cube, but your program does not allow multiple instances.
I can see how the above 2 queries would allow for cheating, however, If it were possible to create multiple grids within the program, the use of multiple grids making HoF ineligibe, that may be a way round things
Alternatively you may consider an expansion that would allow the generation and solving of dion cubes, like you have with rectangles