One thing that's worth adding to the discussion of using the 400 on a monopod is the advantage of using the Wimberley replacement foot. Doing so lowers the center of gravity substantially and makes wielding the rig significantly easier (at least with a light monopod like the Gitzo – the 680 or 681 Bogens might be heavy enough that they balance the lens better with the stock foot, I don't know).
It also makes it possible to use the 400 on a side-mount gimbal head like the Wimberley Sidekick.
The 5540/41 doesn't need to be any larger in diameter, but it could definitely stand to be about 6" longer for us taller folks. I had to have an extension fabricated for mine.
Yes, I'm going to order one as soon as I sell either my 1Ds2 or my 200 f/1.8. But if somebody would buy my White Lightning x3200's, that would help too. ;-)
