For anyone on the edge of her seat (or his seat), and for posterity, here's the scoop: using the potty clicked for Sam on the third day of at-home nakedness. Transitioning to undies at home has yet to click after more than two weeks. A few successes but mostly accidents. I've been letting him take the lead, with gentle reminders from me, but no forcing him to sit. Lots of accidents. Not the easy kind. Lots of laundry loads consisting of one tiny pair of underwear and one tiny pair of shorts. Part of the problem may have been that we ran out of reward m&m's soon after the transition, which was part of our plan (not wanting to continue the sweets reward for too long), and for which we gave lots of reminders (look, they're almost gone) and provided a substitute reward (stickers). Possibly Sam decided that using the potty was no longer so fun without m&m's, or possibly he is just having trouble recognizing the need to go when he has clothes on. Either way, I soon bought more m&m's for the reward jar. Still the accidents continued.
As of today, we do more diapers and occasional nakedness, but a few days ago I quit the underwear unless he asks for it. I was starting to get frustrated with the messes and didn't want to pass on the negativity to Sam's process. Just a little break for me, I suppose. School starts next week, and they will help with training Sam, along with a few other kids, so hopefully the group experience will help him. Meanwhile I'll do a little research and continue with underwear when I get my mojo back.