Not being able to find anything on roots that was suitable, I cut off a long branch from the rootstock and grafted both ends of it, one below most of the damage, and the other above.
Rubber bands weren’t going to hold these pieces together.
I nailed the bottom to the base of the tree to hold it in place, covered everything with parafilm, and tied it in place with kitchen twine, so i wouldn’t put too much stress on it when i grafted the top. I tried to nail in the top, too, but it was too small, and i just made a mess of it. Oh well, maybe that exposed extra cambium. After trying a few more things, i ended up just wrapping it really tightly with parafilm, and then tying some kitchen twine over it.
I guess time will tell.
I got permission from the neighbor to dig out the “small shoot”, but it turned out to be emerging from a beefy root (branch?) running along the rocky ledge it’s growing out of. Also, something ate most of the top growth. 
Not that it mattered, immediately, but that might have been insurance for the parent the.
So… I’m not going to prune out the growth from my stump this year, and if the top dies, I’ll try starting over with a new graft. I think the neighbor’s tree will survive, and there’s now enough of it growing into my yard that i can just take some scion wood from my property. (Not that i think the neighbor would object, since it’s really hard to even get to that corner of her yard from the part she uses. But it’s simpler not to have to coordinate.)