Messy if the pilots use the transfer door/box to pass soiled diapers out of the cockpit. Seriously, there was a female astronaut who wore diapers on a long drive in a car to commit a murder; no need for pit stops that would delay her. So people do use adult diapers to avoid restroom trips.
This is a bit off topic but it is really sad for our wheelchair friends: many otherwise potty trained people have to wear diapers or, more likely, catheterize themselves when they fly, because the on board bathrooms are too small. I find this inexcusable. My own father has to sit in his dirty diaper when he flies because he needs an aide to change him and there is no room for two in these bathrooms. Forget the Mile High Club — this is an atrocity that people like our beloved JimRob here cannot fly comfortably.
In my mind I already solved the problem. Have the furthest rear restrooms face each other in the galley (like many do) and have some fold away doors that can, for a disabled passenger, seal the two bathrooms into one larger room, so that people can use them with an aide, or be able to be helped to the toilet if lower body disability. You would have two normal small bathrooms all the time until someone disabled needed a turn.