IF you were using an aluminum seatpost, you might squeak it in, but carefully, and with some grease.
With a carbon post, it must fit exactly.
Too tight, and the failure of the post will be compressive. Too loose, and you will bite into the post with the seattube and crack it in half. Most manufacturers of carbon seatposts and bars don't recommend the use of shims either.
Just don't do it, and save yourself some pain later.
Thats what I thought, and told the guy, but he wanted us to look into it. So, anyone know where I could pick up a ti or carbon post in 26.4? He's dead set on carbon or ti. Tried talking him out of it, but you know how those guys with deep pockets are...