Hi Scott,
I have succeeded in grafting them. I had three of three take. The scions were harvested by a friend around January or February. I remember thinking they didn't really have any swollen buds, but my friend who has the named varieties doesn't live nearby, so it was my only chance. I used a cleft graft, wrapped tight with a rubber band, and covered with parafilm. I left a few leaves on the plant and made sure to rub off any new growth below the grafts, similar to what you'd do for avocado grafts. Given that I received the scions in an odd season, I assumed they would take more babying that typical grafts, so I put paper lunch bags over my grafts. My plants were in my passive greenhouse and experienced temps from 40ºf-100ºf, thanks to our crazy heat wave in late winter. The scions finally began to push around April. At that point, I moved them outside. They're in the ground now, and haven't grown much all summer.
If I had more material to do it again, I'd probably wait until around April to do the graft. My guess is the grafts would take more quickly, but I can't say for sure. I'm in Zone 9b in coastal central California, so ideal grafting time might be sooner if you're further south.
I hope that helps!
Best,
Reed