Mega,
Go ahead and email/call TMDSAS about your situation (they respond pretty quickly).
I'm in a situation where I applied to all the TX schools except one: UTMB (I'm still waitlisted there). TMDSAS told me that it would be no problem if I just mailed them the extra money (10.00, I think) and photo when the time came to add that one school to my application.
So, it would probably be the same deal for you, but I'd contact them just to be sure.