Can only partially help as I don't have a Mediator but I do run an Indivision AGA in my 4000D with a Retina Z3. The Retina handles my Workbench screen and, if running a game, I hit a button on my KVM, and I'm staring at the game on my LCD monitor.
Works well with my PC connected to DVI on the LCD and my Miggie connected to the VGA input. An Amiga 600 is running composite into the same monitor (24" DELL 2405FPW).
