A FIREd sub-forum seems like a sensible idea to me. I only look at a subset of the sub-forums (Real Estate and Kids aren't relevant to my interests, for example), and I assume other people are the same. As a way to filter out stuff I don't care about, the sub-forums are nice, and I'm sure there is a ton of stuff discussed by the waiting-to-FIRE that the already-FIREd would like the opportunity to filter out. If I was newly-FIREd, I'm pretty sure I'd start spending more time on that sub-forum. And of course like all sub-forums, it would be open to everyone, it's would be guarded by the IRP anything.
It's certainly a better option than "oh, you're FIREd? Sorry you don't fit in so much here anymore, you might want to move over to early-retirement.org". I think it would be motivational to the waiting-to-FIRE population to see the already-FIREd community grow, rather than just see them disappear. Otherwise it becomes too easy to believe the naysayers who say this MMM thing is just a myth, because "hey, hardly anyone at that forum is actually retired!"
Maybe right now the Mustachian FIREd community isn't large enough to support its own sub-forum, but that community will presumably grow with time, so I don't see much harm in creating it; if it gets used, great. If not, no big deal.