LOL Jon, I have seen stupider at halloween. Last year I saw more replica firearms than I can count (city center to boot) and cops didn't give a damn. And god help you if you stray too near the gay bar... skimpily clad blokes galore... (though as a nightclub I must admit it is the cleanest in the city - it is kept immaculate, unlike several others which are FILTHY. Too bad they aint the right place for a straight bloke

). As for the gas mask, a friend did that last year. And the lab coat is knee length...
I often hear that in the US and UK they seemingly arrest one for things that are totally acceptable here. Staggering down the street drunk here is totally acceptable at night, however I am told in the US being drunk in public is verbotten!
Cultural differences... But I digress.
BEST story I heard was of a 16 year old who dressed up as the pokemon 'pikachu' and tried to get into (name witheld) nightclub, a over 21's place that night (normally it is 18+ as 18 is the age to buy alcohol, and that club is THE strict-est in the city) and instead of his ID handed over the Pikachu pokemon card. The bouncer thought it was so funny that he let him in.
The cops will be too busy chasing down either A: Vandals, B: people lighting off illegal fireworks (anything better than a sparkeler is Verbotten!), C: Underage drinkers and D: Drunks fighting.
They honestly won't give a damn about some eccentric dress...
Though I am tempted now to go as John Travolta's character from Pulp Fiction! (last year it was Keyser Soze from The Usual Suspects, complete with replica M1911 on open display, not a single cop looked sideways at it).
A friend is going as a SS trooper with his replica MP-40. I think we are filming a follow up to "Hitler Can't get into CP's" at CP's (youtube that video, its HILARIOUS! It refers to a few 'dives' in the city, that let 15 year olds in. CP's wont let anyone in!)
heres the video...
http://www.youtube.com/watch?v=fUJ245F9bmg&feature=related