CrossRoads:
What if you use a Mega instead? Then you'd have plenty of PWM pins to support 1 pin/unit and not need any external hardware.
Even if all these rooms are 'close' together, I have to wonder what kind of electrical issues you will run into running wires dozens of feet and putting an IR LED on the end of it. The capacitance of the wire may slope the edges of the 38kHz square wave enough to make reception unreliable. Or it might work just fine. I dunno, just a thought.
Microcontrollers (the chips used in the Arduino) are not expensive. You could, for example, use an ATTiny85 chip (which costs less than $2/ea) at each aircon. A 10-pack of small perfboards to mount them on is not expensive either.