Generally, I'm in FC so it's not an issue but I have used the FC lavs on occasion when I'm near the front of the plane. I just stride purposefully to the target door.

Seriously, though, while I'm all for keeping the unwashed masses in the back of the bus, it's a common sense thing that if you gotta go and the aisles are jammed with carts, FAs, kids, etc., then I don't see anything criminal with using the lavs up front. What I've done is wait until I see the lav is empty and make a bee-line for it. Never been stopped so far.

Granted, it's easier to enforce a "no exceptions" policy because you don't want a half-dozen people from the cheap seats milling around the rarified atmosphere of FC, but there are also times when someone really has to go badly. Always exceptions to the rule, I suppose.