I read your blog and also the Magic cafe posts, and I was thinking that if everyone loves the diaper ideas so much, maybe have a small inflatable baloon in the diaper for when baby makes 'that poo-poo face'- especially if you accept the one posters' idea about the fart machine.
This would also work well if you have an audience member wear it too- just have them make a face, and imagine the fun you will have seeing THIER expression as the diaper 'fills'!
Stinky, but funny!
What about using an old trick with a new twist? Remember those 'disappearing milk' glasses? You pour the milk into a rolled up newspaper, and the milk disappears? Why not do the same thing, but turn it into a baby bottle? The vent will still be able to talk, because there is no actual liquid going into 'the baby's' mouth!
What do you think?