Took me some time to find out that RealTime provide devices to interact with Daikin SkyAir indoor units such as mine. This RTD-10 device talks ModBus over RS-485 links, so I also bought a USB to RS-485 adapter that works.
Thankfully, the RTD-10 comes with two cables: one that connects to the indoor unit 16V power supply (X35A undocumented connector), and one to connect the P1P2 network.
Once all was connected, I plugged the USB to my closest Raspberry Pi, [ab]used the unit test client from libmodbus, and quickly got the on/off, setpoint, fan speed and mode information, and could update it too. I can also read the Daikin remote control temperature reading.
HNet was not really ready for ‘system’ integration, but after a few hours I could read and change all settings from my openHAB application (HNet generates the whole configuration for openHAB 1.x). Of course it is now possible to set some integer to text mapping as “mode 1” does not help much, so we have mode heat (or chaud in the French openHAB sitemap).