I went ahead and finished them on the kamado. I recognize I'm not getting anything out of the smoke at that point, but I figured the kamado environment would be moister than the oven. And I was afraid re-heating in foil might ruin the bark. And my oven is inside, where it isn't sunny :-)
Took about 4 hours at 260 to finish them. Temp went up pretty linearly, no stall. Cooked them to 196 and 198, foiled them and let them rest in a cooler, hopefully to reach 203. But i didn't want to risk keeping them in there any longer than necessary.
Result was fine. Family loved it and I would have proudly served to guests. One of them came out almost identical to typical results i got on my previous akorn, the other was maybe a touch drier. Both with a little thicker bark, probably from the longer smoke time at the lower temp the kamado joe can sustain vs the akorn.
The one being drier was probably only noticeable to me because i was paranoid about it. With some eastern NC vinegar sauce, it was certainly indistinguishable. Not sure whether the drier one was one pulled at 196 or the one pulled at 198.
Anyway, if anyone stumbles across this post again - I wouldn't plan it this way, and the best thing to do would have been to just finish them in the oven that night, but at least this proved that it can work out fine to finish them the next day. Pork butts are a good forgiving choice for a first cook on a new set up!
Appreciate the tips - look forward to coming back to the forum often!