I'm actually a pretty big fan of this, though I know it's a niche within a niche. The thought of someone being completely immobilised, and left to use their diapers by necessity is definitely neat, though it’s really just another form of bondage.
Here and there on various sites I’ve done RP’s involving casts, and I’ve actually tried incorporating them into a story recently, though it’s hard to keep it interesting. Usually my fantasies with it tend to get kind of outlandish ie ditzy nurses making life hell for the restrained patient, said patient being treated like an infant since they’re unable to complain (People cooing and baby-talking to them, reading stories, turning the tv to infantile shows.) Or being downright malicious, and doing things like changing their chart to arrange for a massive dose of laxatives or postpone a change.
Anyway, I like it tons. Big creepy post over.