Ok. These are annual events at Portland:
1) Santa bar hop- I've seen this myself. Hundreds, and I mean hundreds, of people dressed as Santa go bar hopping. It's load, and it's massive. And all they do is drink.
2) Nude Bike ride- This happens during Pride Fest (which is a weird event in its own right). People get naked and ride bikes. Not much more to it.
3) Nude Bar Marathon- I learned this from Chuck Palanhuk's (spelling, sorry) new book. I've never seen it/done it, but there's something like 44 nude bars during this time, and the goal is to go to each one of them and have one drink while there. No one's made it past thirty. I guess the top numbers are somewhere around 22, which is an enormous amount of liquor.
I'll make a note about Pride Fest, even if you never actually see the festival, you know it's going on due to all the drag queens wandering around. Some of the outfits are ten times worse than what happens at Kumoricon. Seriously.
I think everyone's right. No one will care if you're in costume on the Max, but I will say that groups make you less self-conscious.