The only issue with grafting pear to hawthorn is that most pears will grow faster than the hawthorn. So at some point, the pear might be so big and heavy that it might break at the graft union. But if I had a bunch of wild hawthorn, I'd be ok if that eventually happened to some trees and you would be able to regraft those. I think if you bark graft to larger diameter hawthorns, you should have a few more years before the top outgrows the hawthorn rootstock.