B2RPW,
I also used Joe Salvatto to install my studio with a DM4800 and an Argosy desk. I had the same LCD problem and installed a 2seemy which did NOT require to take the desk out of the Argosy as I was able to pass the cables and mount the card with the mixer in position in the desk. A bit tight but doable. As far as desk space, I simply bought a cheap VGA switch box to have my monitor display either my computer or the 2seemy. Now, like you, I am on MacPro for DAW (Logic to DM4800 via FW card) but I actually also have a PC for my guitar switching control, so I put the VGA switch box on the PC monitor. However, I see no reason why it wouldn't work with a Mac monitor, as long as you don't try to run the 2seemy in software mode, but simply as a direct hardware connection to the monitor.
The 2seemy on a 15" monitor is awesome and I tweak my DM4800 much more than I used to. Wayyyyy more powerful than any Presonus (which I know well) and you don't lose the control surface. Not even close...