as for hardware broadcasting of keys... The Vetra was what I was thinking about and it requires a serial or PS/2 keyboard...
your setup would have one serial keyboard sending the same keystrokes to all of the PCs it's connected to and seperate USB keyboards on each PC so you manually control just one PC ( if you think you need manual controll on each )
CoH does not run in seperate windows so you can't run multiple copies of it like you can WoW ( or at least that's what I remember about it , I know I couldn't get two copies going at once on one PC )
which limits you to having a seperate PC per copy of CoH

Several people here do the hardware mboxing thing, I'm sure their 5 box setups could run 5 copies of CoH.