This is very cool. I have a possible use case for this: spaced repetition for learning new vocabulary. If you could easily generate crosswords for the words someone is learning (plus some filler words that the learner already knows), it would be much more entertaining than reviewing Anki cards.

I think the constraints are more relaxed for this use case. The grid really doesn't need to look as neat as an NYT crossword puzzle.

I bet you can already market this to ESL teachers, and perhaps make some $$$ off your hobby project. Good luck with that! (Of course, I mean it sincerely, not ironically :-))