I think a shim would work OK if you bought a 25.4 to 26.8 shim because it would be the correct thickness-i.e. 26.4-25=1.4mm difference in overall thickness. Just make sure you get the kind which is split down the side or you won't be able to get it into the seat tube. You may also have to trim a little metal from the split.

http://www.niagaracycle.com/product_...ucts_id=417844 Here's a single shim that makes 1.4mm.