The reason I asked the question is if Imodium can mask pouchitis systems can pouchitis untreated cause the pouch to deteriorate and fail?
Imodium isn't anywhere near powerful enough to do that. All it can potentially do is slow you down (and it probably won't be of any help with that in the face of virulent pouchitis, anyway).
When pouchitis is really bad your symptoms can be wide-ranging and disabling. Diarrhea's just one component of it. Even if there was a drug strong enough to mask its symptoms (as opposed to actually treating it), the inflammation would show up in your scopes.