I used a USE thermoplastic shim to fit a 27.2 on my 31.6 on my Pegoretti...worked great.
These are machined to high tolerance from UPVC. They're 100mm long so lots of support and tight fit. Because they're made from a plastic, no need to worry about seizing a carbon fiber or ti post. Might even add a bit of dampening to your ride, but that might have just been from switching to a smaller diameter seat post.
Note that their thinner shims are aluminum and I have no experience with those
http://www.ultimateuse.com/use-components/accessories/309mm-id-shims