A little alarm is going off for me on this one. Had you said quince I'd say it was worth a try, but I think hawthorne won't work. Still, why not try? Perhaps with the right interstem ... ?

Joe Real used to come up with some remarkable multi-species grafts, if you can find any of his old posts. Been a few years, though, and the search engine here can be frustrating. Scott Smith has seen delayed rejection sometimes, so it can be a long time before you know for sure. He might be able to tell you for sure.

I'd be interested in hearing the results if you give it a try. I did stick a prune onto Nanking cherry a few years ago, and it's still around. But that's a pretty close relationship, and I don't know if hawthorne is close enough to apples and pears, although apparently it is (to my surprise) a pome, albeit a single-seeded one.

I've done a few pears on native cockspur hawthorn(C.crus-galli) understock, just for grins.
Have one surviving(other was fine, but was removed when my wife put in a tennis court) that's been in place for well over 10 years. Seems to be quite dwarfing - this last tree is only about 8 ft tall, and almost like one of those 'columnar' apples.
Have not needed to stake this one, but the other one exhibited significant difference scion/rootstock diameter - scion variety 3" diameter after a few years, rootstock only about 1", so I staked it...

Have not tried apple on hawthorn.