Can you elaborate? Are you referring to the minimum seat post length that must extend past the top of your seat tube? If so, what is the distance you're trying to work with.... measured from the top of your seat tube mast to the seat rails?
Lynskey R230/Red, Blue Triad SL/Red, Cannondale Scalpel 3/X9
Totally unrelated, but being the captain of an AL-framed tandem I didn't have the option of putting a suspension seatpost on my seat, so I just put a Wound Up carbon post up in the captain's seat... I'm amazed at how much it takes the edge off of rough roads and even the smaller little bumps... I have numbness problems at the drop of a hat, and the first ride with the Wound Up post was the first time I wasn't even a little uncomfortable.
Very few upgrades out there where you get more comfort and shave 1/4 pound off the bike... Just another option if you don't have room for the suspension post--and even the high-end carbon posts are the same price as shock posts...