I have a built a home theater in a room upstairs. My house currently have two HVAC systems... one for the downstairs and one for the upstairs, each controlled by it's own thermostat. The theater is an inside room upstiars and with the equipment and 5 people, it gets really warm. I don't see a need to heat the room as it's already heated by the existing HVAC system. However I need to figure out a way to idependently cool that room exclusive from the rest of the upstairs... so was thinking about whether it's possible to wire a thermostate to a portable AC unit (I would but the A/C unit up in my attic). Any ideas are apprecaited!

This answer probably has a lot of "depends" to it. It might be best to pick a unit out and either get a schematic/manual and or check with the manufacturer.

If the system is a simple knob thermostat, then probably it's no problem. If it's digital and has buttons, then "maybe". Because, you might have to "trick" the sensor to constantly thinks it needs cooling and cycle power through an external relay. Providing cyclng power doesn't turn the unit off.

A stupid way might even mean pointing an internet camera on it and interface to the buttons. I can come up with an isolated reliable way of interfacing the buttons. You'll have to move the sensor as well.

Another way might be to trick the sensor being used that it's too hot. Sensors might be either silicon band gap or a thermister.

Who knows, the system might have that as an option.

One of these guys has a remote control. http://www.amcorgroup.com/asia/download/Amcor%20brochure%20October%202006.pdf

Probably IR. Sometimes the temp sensor is in the remote.
So, all you would need is an IR extender and possibly move the sensor.

Just ideas.

You might look at mini-splits as well.

You might want to re-think this one. A remote t-stat is no problem, but the heat in the attic might be.