For those keeping score. The item that epass has offered up to me would seem to be a Power Support RS-11.
http://www.pawasapo.co.jp/products/mac/rs11.php
While made by the same company as the "G5 Skateboard" and the updated Power Mac support devices it is in fact designed for a G3/G4 tower. However I think I can use it as a base and build up a support structure for a Mac Pro.
Long term plan is still to build a Geek Desk I think.
This surface, with a udnder-desk cpu mount...

On this motorized frame (larger then the last one I looked at, supports up to 375lbs)...

But thats a >1000$ project that I'm going to have to wait a bit for.