[squarefoot]

I'm sure that they made things more difficult by employing proprietary hardware wherever they can (also to discourage competition), but yes, there are a bunch of sensors and actuators in there and any board with the appropriate i/o capabilities should be able to interface to them, however writing a working firmware would be next to a nightmare: how do you find developers who want to spend months reverse engineering an AC and also know enough about ACs to put together something that works? Replacing household appliances brains with open counterparts would be a heck of a business opportunity to revive or prolong the life of dead/obsolete products, however I guess finding people who are interested enough to do that with FOSS, essentially selling only hardware and installation services would be really hard.

[datavirtue]

It's just a pin out interface controlled via software to turn things on or off. Its trivial. Get a raspberry pie, lookup the pinout docs stuffed away in your home manuals drawer, and write the measly logic required. The most difficult part is whipping up a UI and building the scheduling logic, if want/need it.