All the HVAC control systems are anti-opensource. They pretend like their proprietary controls are trade secrets worth billions in research and development, but ultimately they are all just glorified mercury switches. Honeywell, Johnson, Mitsubishi, Schneider, Trane, Siemens, none of them want to allow third party control without getting their beaks wet with licensing fees. Even their commercial departments have started phasing out support for protocols like BACNet and Modbus.
Temperature sensors are cheap as shit. Low voltage relays are cheap as shit. Even digitally controlled zone dampers shouldn't cost more than $100 installed. If you can access your ventilation in your attic or basement, you could zone every room in your house for less than it costs to replace a single AC compressor, and run it all on a raspberry pi.
But you need to know what you're doing, and they will throw every hurdle in your way. No contractors would risk drawing the ire of their suppliers by doing it for you.