I usually just put a very short "noteworthy" section at the top of mine and highlight it with asterisks for such things.
Personally, I got tired of doing the role played mask. I found that for me, its just better to have my character do things via proxy. Ie. If you have a thief character, then try to get the things they intend to steal by involving another innocent character. Have them "Borrow" what you need, and then "borrow" that item from them, but don't give it back or give the illusion that it was given back so that someone else gets blamed. It's a little hard to explain and takes a lot of thinking on your part to pull off something like this.
Some of the benefits of doing bad things via proxy are:
1. Your friends are no longer stuck with the burden of dealing with your mask and remembering that they did not recognize you.
2. You leave a trail that could be traced back to you. Remember, if you commit the perfect crime, then you kill a portion of the RP for for yourself and everyone else. If you are a baddie, its OK to get caught.
3. When you do things "masked", you sort of kill some of the mystery and surprise for fellow players.
I think using masks in RP is valid, but there are better ways to pull off such things.
As for requesting masks, I know this has been requested several times in the past. One possible workaround for the time being would be to use a helmet to signal to other players that you are masked. It's not a great option, but it might be better than nothing at all. ( though a special "mask" helmet that looks more like a mask would be nice.. that and cloaks for all models )