You could try to do what is necessary to install Leopard Server on the new Mac Pro's, which is to install Leopard Client, do all updates etc, then put in the Leopard Server disk. Can't remember if it is the main installer you click on or if it is in one of the folders on the disk, but you can install the server components without having to boot off the disk.
Once done, do another software update to bring the server components in line with your leopard updates, then reboot
I discovered this when I bought a new Nahalem Mac Pro for a client with a copy of Leopard Server. The Nahalem requires 10.5.7 (I think) to boot, but the latest Server disks are only 10.5.4. Very simple and straight forward to do, just a bit frustrating.