Hmmm, interesting question.  Most of the guys I've interviewed have gotten 
dressed up even on casual days, but then we're pretty causal every day so 
it's hard to tell.  I don't think he'd mind if you called to ask.  That 
department is a little dressier than my area (they actually wear ties most of 
the time Mon - Thurs) but I think Friday is pretty casual there - and you 
have a pretty good excuse.  It might depend on who else (if anyone) you'll be 
seeing.  The reason they get dressed up so much is that one of the senior 
guys (don't know which one, but can find out if you want) on that floor is 
always in a suit & tie.  If you're also talking to him it might make sense to 
bring a change of clothes and use a stall after the interview.