I was thinking last night, sure he still doesn't walk, but he could sit on the potty, just to see what it's about, right? So before bathtime, I pulled it out and sat him down and sure enough, he did it! Total luck (on 2 counts, 1 for doing it in the potty first time out, 2 for not doing it in the bathtub). He asked for a magazine and he likes to wipe and wash his hands, so even if he doesn't repeat the performance for a while, at least he likes the process!
We tried it again late this afternoon and we are officially 2 for 2 with number 2 :). I am not sure why it's working, but I am grateful. If we make it 3 days in a row, it will be a real feat!