Does anyone know if this could be used to "cap" the AC settings ? It could be useful in an airbnb place, where I want to limit the lowest (e.g. 18 degres Celsius) or hottest (e.g. 22 degres Celsius) the guest can set the AC.
To answer your question, yes you could certainly do that and override what ever the physical thermostat was set to. You could also set max hours per day (kWh)for usage or adjust for variable rate plans. As with anything you'd need to test.
Edit: Looking through the repo and the article I see nothing blocking you. You certainly won't break anything!
If you controlled the AC temperature remotely it's a great red flag to never rent that Airbnb. Some people like it cold when they sleep.
These panels technically lets you do everything you would ever want to on an AC control panel: https://www.mitsubishielectric.co.jp/ldm/slim/remotecontrol/
Or multiple AC units can be networked and managed over Ethernet: https://www.mitsubishielectric.co.jp/ldm/control/function/
People do start placing hand warmers on the panel if you lock them down too much, but what you asked should be possible in principle.