Thats a hard one.

When DD was little (well 16months) I could predict her poos and couldn't be bothered with smelly nappies so I'd rush her off to the toilet. Eventually she would hold on to them until bed because she was being lazy!!! But that's not what you're talking about.

If it were me I would leave her without a nappy for her nap and let her get really wet, hoping she'd decide that is yucky. Chances are she is only waiting til you leave the room anyway and it won't actually wake her up to wet the bed. It means messy sheets, but I don't see other alternatives...

Oh and I found with DD that TT took off when I took her night nappy away (never had a nappy for her day nap once we TT).