Steve,
I have grafted/budded peaches and Japanese hybrid plums onto Nanking cherry rootstock(it's actually more closely related to plums than to true cherries). It's very dwarfing, in my experience, and to tell the truth, I just did it to be playing around. None of those grafts are still alive today, but neither are the Nanking understocks. I'm pretty sure the peach did bloom and produce a few fruits for my dad, but central AL must not have agreed with the P.tomentosa, 'cause they all died out after a number of years.

Steve-

I grafted a prune to tomentosa two years ago and it isn't dead yet (I'm in Montana). But as Lucky notes it does appear to be very dwarfing. The first year the scion shot up 12 inches, the second it produced secondaries, and right now it looks like it wants to stay right where it is.

This could take a while ...

Good luck,

Mark

I read somewhere that you can use peach on Prunus tomentosa as an interstem for apricots. I don't remember where I read it, but I do remember that the experiment was on a fairly large scale. It was successful for at least 5 years if I remember correctly.