'Chetopa' shellbark hickory is reputedly a genetic dwarf. The ortet (no longer in existence) was bearing heavily as a 6-8 ft tall 'tree'.

I have it here on pecan understock, which presumably pushes more growth than if it were on hickory understock, but I'd guess it might be the most promising candidate.
You could probably get it as a custom graft, from England's Orchard & Nursery, if you're interested.

reason I was asking is I been looking to do some functional ornamental stuff, and I was hacking through the brush near the back and I found like 5 hickories packed together (most likely from squirrels burying them near each other. They're too close to let them all grow, but I really don't want to kill any natives Just wondering about their resiliency with having their leaders pruned back to develop cordons.