With only the G15, no. It blasts everything - you would have to set each machine that gets the data to selectively interpret the data differently.

Plus, the G15 is USB only - a big problem when it comes to multicasting. You will have to go software only - but the good news is that it IS possible to do what you want it to do. It's just tricky.

Either remap the main system to ignore said keystrokes and use synergy with broadcasting to send them (if it will, the extra keys can be tricky) or try out multiplicity as you already suggested. Another option is to program the G15 to make the extra keys perform as "regular" keys - like perhaps alt+1-9? I avoided the G15 because it was USB and well - HUGE so I cannot say for sure if this will work or not.