I never updated that thread with my results. I grafted 2 kinds of quince and some medlar onto the hawthorn, at least 2 sticks of each, and all took. It’s only been a year so far, so I can’t speak to long-term compatibility, and the quince does seem to be growing wider than the host, but those medlar bore the same year, so it was at least a partial success.
I’ll probably try pear this year just to see.