I think potty training for ABDLs is a flexible concept and most of us could find something to enjoy around the idea. For example, a great and very simple thing that makes me feel little is somebody else asking if I need to use the bathroom. If someone who was in the know asked me if I needed to go potty (and used the word potty), I'd feel very small and it would be great. They wouldn't need to follow through or commit to some big RP scenario or anything, just asking if I need to potty if I haven't gone in a while would be awesome.
On the other hand, there definitely are more elaborate scenarios that could be fun and that some people have tried out. Things like planning out a set of rules in advance with a partner, wetting when not wearing a diaper and then being put back in diapers, or even going through a days-long scenario where you get a gold star for every trip to the bathroom could all potentially be fun with the right setup and the right person engaged with it. They are also harder to get going and could fall flat though, so I think it's really a question of the interests and fun of the individuals that would be involved.