In Otakon, at least, it seems that walk-ons are almost pointless, unless one wanted to have their own music or short dialogue. That's because there's a separate hall costume contest. Plus, all those who participate in the Saturday hall contest are required to go to the Masquerade and participate in a 'fashion show,' which is practically a walk-on. *shrug*
And I've never heard of an "Out of Competition" category for cosplay. Why would anyone want to spend so much time working on something that wouldn't be in a competition?
Halloween '07: Toby (Sweeney Todd)