There are no archive sites associated w/ FA that host adult HP fic, but there are several non-affiliated ones that do host NC-17 fic. restrictedsection.org (all ships), checkmated.com (Ron/Hermione only) are a couple I can think of off the top of my head, and Sycophant Hex (can't remember the url, but it's the first thing that pops up on a google search) has branches that host different genres/ships of fic. I wouldn't recommend any of those as paradigms, since they all require registration to see teh pr0nz, but as a guide on how not to do things, or to see their categorization schemes, let me know and I'll let you borrow my passwords.