I posted a lengthy and detailed response that apparently hyperspaced. Anyway, Rob's got it right, and if you want to do this via IR, I suggest using two Xantech CC62 IR-controlled relays to provide the 12 necessary contact closures for all four zones (3 per zone: On/Off, Vol+, Vol-). I would favor this solution over that proposed by Impaqt, for it is not only cheaper (though still pricey), but will turn the amp on/off as needed.

Good luck.