Sounds interesting... but couldn't they (between Midna, Zelda, and Link w/ master sword) try and remove it? Or would that be the 'quest' perhaps?...
Anyways... why would Zelda end up diapered?
I think she might have an issue with her magic, perhaps a mistake while traveling through time, or practicing with some of it leading to trouble. Could even be that she suffered an injury from being possessed, maybe Link accidentally injured her while trying to free her?