Of most of the clubs and hobby associations that include costuming as part of the hobby activity, just about -anyone- tends to be well received at any convention which includes hall costuming. I've been going to SF cons since the '70's, and I cannot recall anyone at an SF con being told that the 'theme' of your costume doesn't 'fit.' (Complaints about indecent exposure are an ENTIRELY different subject... and let's -not- go there.)
The SCA is a medieval recreation society, and often those folks visit SF cons in perfect 1400's garb, but no one hassles them for them -NOT- being SF. I would think that just having fun in a costume will elicit a positive reaction from an anime con, as long as they recognise the character. You would get a smile out of a perfectly well-done "Beetlejuice" dude ("It's SHOWTIME!") because some folks will remember the character and his schtick.
You can always play it as "hey guys, I'm just crossing over to visit from the parallel universe of another genre - how is it here in anime-land?" Or a Tom Bombadil could try "Hey, I was cut out of the movie, so I ended up HERE with you guys!"
My first Kumoricon, I saw an 'Invader Zim' costume, and my thought was 'Hey - theat's -NOT- anime, but it's FUNNY." So, on the one hand, I would hope that no one with a non-anime costume gets hassled, -BUT- on the other hand, I would hope that the costume judges hand out awards within the theme of the con. I think it would be really weird FOR AN ANIME CON to have a decently-done Gundam mech get passed over for an award because somebody else did a more perfect Star Wars Storm Trooper, or a Star Trek uniform, or a Warner Brothers' or Disney cartoon character.
You might give the 'outsider' some kind of honorable mention though, if the costuming is so superb that most other members have a positive reaction to it, -OR- it's a character with a parallel relationship to some other series which -IS- an anime. Like, if someone did a cosplay skit with Harry Potter, and some anime characters then they could all get an award as a group. A better example might be mixing in an LOTR character with a bunch of Lodoss folks, or a skit like "Let's go questing with Sam, Frodo, Bilbo, and err, LINA INVERSE?!?!?" or 'Presidential Debate 2008: Zechs-Vader vs. Beld-Sauron. Arshes Nei is our moderator..."
OTOH, I was at Sakuracon, and even though I -have- lived in Japan, and -do- speak some Japanese, I *despise* dubs and just about never watch anything if I'm not hearing Japanese, yet I -still- found the J-rockers sort of catty and aloof. My reaction was, "Hey, that's the way they are - it's part of the character," and since I was there to have FUN, I just hung out with other folks instead. So I don't think it was a 'hardcore anime' vs. 'cartoon network wannabe' elitism, maybe it's just THEY'RE LIKE THAT. Deal.
- G