Can fruit pears be grafted on ornamental pears such as the Cleveland and others?
Yes. And it will considerably improve the ornamentals, in my opinion ...
Several years back, I taught a co-worker to graft, loaned her a grafting knife and gave her a mixed bag of fruiting pear scionwood. She grafted just about every branch that she could reach from the ground, in the 10-yr old 'Bradford'(or whatever it was) in her backyard. Her pre-school boys delighted in showing folks those random branches with big, tasy pears. But, eventually, the tree fell apart, as callerys are wont to do.
I'm constantly finding more and more 'volunteer' callery seedlings around my place, courtesy of the birds and the myriad 'ornamental' pears planted all over the countryside. I flag them and either graft in place for later transplanting, or dig them, transplant or pot up, and graft them over to fruiting varieties.
Some callery seedlings have pretty decent branch structure, early on in life, but I suspect that if left to their own devices, they'd assume the typical 'Bradford/Cleveland' habit, and eventually fall apart, so I try to make sure that the apical portion is replaced with a fruiting variety, and re-work lower branches that I leave in place fairly close to the trunk.
Have several large seedlings that I've grafted over to late-maturing Euro or EuroXsand pear hybrids to plant out on the farm as 'soft mast' trees for wildife.
lots of good info there.