Seatpost
I`ve taken a post from 27.2 to 26.8 before. Only a cheapy post with plenty of wall thickness mind you. Lightweight posts are a different territory as they`ll probably break if you take off more than a whisker. If you have access to a lathe its an easy task, about 15-20mins work although a machine shop would probably charge you the price of a decent post to do it.
Is it a type of post you want to stay with or is the frame a weird size?