This may be of some use to you. (
http://www.spookhouse.net/angelynx/comics/champloo-music.html)
It doesn't look like it's on any OST, but the site answers the question.
It's a remix/riff based off of Affirmation by George Benson.
This raises the question: Does an unreleased tracks edition soundtrack exist anywhere out there, like I found with Ghost in the Shell: SAC on this site?
There's a bunch of great music from this series I've never been able to find, either.