Well, so far it seems that the numbers being entered are not sufficient to warrant separate categories and they seem to be 'display only' at this point. I do see that changing over time. I can see them initially being entered in the 'Painter' category if they become eligible for awards. There is a valid argument for entering them in 'Open' since they are essentially scratch-built. I'll leave that argument for the philosophers...
Putting a size limitation on the entries is a good idea (the pic I added below is about a two-inch/5 cm disk), otherwise things could get out of hand. I'm not sure the under-surface is relevant. People get creative, so painting on canvas, illustration board, wood, ceramic, stone, and yes, eggshells, would all be fair game. What would be relevant is the subject matter - people. Historical, sci-fi, fantasy, civilian, cartoon, comic, etc. - people.
I think the best place for them (for the moment) is the generic figure show: SCAHMS, MFCA, MMSI, etc., as well as the equivalent British/European shows and the World Expo. After all, they all have Ordnance categories for aircraft and armor, and Diorama categories which can be people, machines, or both. I think genre specific shows, such as Kulmbach (?), would probably opt out.
I think I would like to take a crack at one of these - and I know just the person to ask. I have Jessica's email addy...
Glen