Considering Blizzard's supposed greenlighting of Hardware based broadcasting your best bet is likely a 2nd KM Switch that either:
-A) accepts macros from the mouse to switch between all or specific PCs/VMs;
-B) Has a way to accept input from your keyboard to have your mouse to switch between all or specific PCs/VMs;

Synergy is software broadcasting of inputs and able to go to multiple clients on multiple machines and may be limited to just one machine at a time though I would hate to have to try to argue/prove that to Blizzard. If you do look at Synergy you may want to look at the free Open Source fork of Synergy which is named Barrier if my memory is correct.