I'm pretty sure I remember reading a blog post (maybe by Rick Seaney over at Farecompare), about there being no requirement for working lavs. I know I've definitely done an express flight (~1 hour) that had the only lav inop. I think it was a CR7. The GA announced that the lav wasn't working prior to boarding, so told everyone that if they wanted to go, they should use the airport lav prior to boarding. But that is certainly a completely different situation then something like an LAX-IAD flight.