Karl;
The only problem with the layout in your picture is that it puts the scanners in the middle of the long edge of the projector. Once you send the beam 90 degrees left or right, you've got a long way to go to get to the output aperture at the edge of the baseplate.
This is a layout I've been toying with if I ever re-build my current RGB rig. It dispenses with the floating tables idea and uses a second bounce mirror (just like your layout) to make alignment a snap. But the beam ends up hitting the scanners right at the corner of the plate, and once they make the 90 degree turn through the scanners the beam is headed out of the projector almost immediately. (This allows for a small aperture window.)
Adam