Heh... my credentials include AD&D, GURPS, on forum and on irc RP...
Just a word of warning here, having the characters of the book involved can be volatile. I was scanning the above posts and see claims to Rowling's characters, and something about an argument of who a character is going to kiss.. I can envision that issue coming up even if only one person does have a definite claim to a certain Rowling char. And then there's an issue of whether or not other people felt the character was being played appropriately... and I just see a lot of possible problems. Personally I can see all the original characters you guys have come up with being strong enough to stand and play out a storyline on their own without involvement of Rowling's cast.. or they could come in as NPC's (non player characters) controlled by the GM, only popping in to help move the plot along and give players clues.
It's just a suggestion, mind you. And probably weird coming from someone whose trying to play Shan Yu over on the animated RP board, but every post i'm terrified someone is going to scream at me that I'm playing him wrong and if that does happen I think I'd be inclined to drop him because it just isn't worth the argument to me.
I wouldn't soeak up but it seems that a lot of people are very attached to being attracted to certain chars, and I'd hate to see flame wars break out or arguments or anything because it really is just plain impossible to have every original fan character that wants a romance with one of Rowling's characters to have them in RP, unless Rowling's cast is a bunch of guys who'll date dozens of girls at a time...