
Re: Graft Candidates for Ash Roots - Ash Borer is Here!
“Are there any fruit trees that grow in zones 5 to 6 that could possibly graft onto an ash?” -- I can safely, if sadly, say
no … there's species incompatibility.
“After a few generations of borers perforate the cambium, the canopy dies or the tree falls.” -- Didn’t know that… So, you could simply allow them to ‘re-fill’ their voids in a rotational die off..? We’ve lots of multi-trunked Big Leaf Maples ‘out here’ where the stump was left after logging and the roots also sent up multiple shoots --which over time became ‘trees.’ Fortunately, beyond further logging, they’ve not had to fear an instant insect attack...
Once had a friend beckoning me to graft ‘replacement’ shoots to the tops of trees lining a yard that the landlord had ‘hacked’ off too short. He’d even saved scions. I finally convinced him that I couldn’t do one tenth the job nature could/would do as those trees each sent up a gazillion shoots the following spring … which they did! In your case, you’ll just be ‘stuck’ with the original, or rootstock cultivars.
“I've been in-planting like mad with many many trees, from small bare root stock to large root-ball stock, but”… -- That in-panting sounds like your best plan for transitioning to fruit trees …baring the rotting corps’ of the larger trees don’t crush them on impact

In fact, you may want to use a NW logger’s trick of coating the cambial layer with herbicide after removing the upper portion of the existing dead and dying trees to kill the roots… In your case it would cut down all competition with your new fruit trees …and you could even plant some very close to the dead stumps.
All that said, I’d hate to stop your experiment! If you’ve any shoots currently growing from the rootstock (or base) of your larger trees -- they’d be
excellent candidates for ‘Bud Grafting’ in a few weeks. It’s a very simple process in which you’d have nothing to lose. Just collect as many different varieties of fruit or nut producing ‘bud sticks’ as you can and
bud them to the multiple root shoots. If any take – great - go from there
! But be aware, though I don’t expect any success… if some did take, there’s a graft incompatibly concern over time; whereas the union eventually fails for various (if usually unknown) reasons.
It would be wonderful to find such a match …but concentrating on safely removing the massive dead & dying trees while in-filling and protecting your new standalone fruit producing cultivars seems the best plan
