I'd say this went from a seemingly stupid thread to a legitimately interesting one.
"...feces contain urease which catalyzes the conversion of the urea in urine to ammonia which irritates the skin and can cause painful redness." - Wikipedia
If urease can catalyze urea, it may be able to catalyze an indicator compound. Or perhaps, assuming one's diaper contains urine, there's a way to take advantage of the urea reaction to trigger an indicator. Or something along those lines...
Either way, wheelchair cases aside, checking an indicator (which would have to be done frequently to avoid rash) would be almost as troublesome as checking the diaper itself. I'd go with what Darkfinn said about regularly scheduled changes.