We don't typically dress up, but I've always been tempted for Halloween to go in to work dressed like a classic IBM technician. White shirt, black pants, black shoes, black tie, pocket protector (maybe even track down a slide rule,) slick back the hair (have to let it grow longer though.) Stays within the dress code but somewhat recognizable.
If it were any other sort of character, maybe Gordon Freeman from the Half Life game. Quiet, stoic, but drops in and fixes problems no one else can solve (often with percussive maintenance)