DS is 22 months old, and has happily taken to the potty, and as of this week insisted on using the toilet instead. Two days out of three are completely dry, and the 3rd day there may be a tiny trickle as we have not managed to get to the toilet quick enough! We still use a nappy for naptime (he sleeps 2-2.5 hrs) and for overnight. He currently only poos overnight or during his afternoon nap, so has only actually pooed in the potty twice and has had 2 poo accidents (v early on, one when I was grabbing a shower and the other at nursery!).
How do I persuade him to poo at other times so that we can ditch the naptime nappy (which is usually otherwise dry), or do I have to wait til he drops the nap?
I think you will have to continue with the nap and night nappies. I don't see how you can 'persuade' a 22mo to poo at any other time than when his body is ready to. Obviously, while asleep he is relaxed and not consiously pooing on purpose. It depends when during the nap, as soon as he falls asleep or when he is due to wake, that he poos, you could possibly delay putting him down for a while, or put him down sooner, and try him on the potty at either of those times.
For 22m he's doing incredibly well, I wouldn't be inclined to mess him about for the sake of a nappy.
Actually, although of course there is no hurry as he is very young, I'd definitely advise you to just stop using the nap time nappy. It sounds to me as if he doesn't like pooing on the potty and is therefore waiting for the nappy and doing it when he has one on. If you do stop the nap time nappy you might have a couple of accidents to start with but he'll soon realise he doesn't have the nappy on and stop pooing then and it might give him the push he needs to start him off using the potty. I had similar experience with DD1 (she was about 2.3 at the time so a bit older), she always pooed at her nap time in her nappy. So i stopped the nappy. First day she did a poo in pants in bed but of course that never happened again, and as she couldn't always hold her poos all day until the night time nappy went on it gave us a chance to start catching some poos in the potty and it wasn't long after that that pooing in the potty became the norm.