Good morning Stig,
I’ve built an addon for SmartG4Control system, it uses a very similar API to what HDL uses, to be honest i think SG4 is just a branch of the HDL protocol. Might be interesting to share ideas as we build out.
See my post about it on the development forum.
https://community.openhab.org/t/new-smartg4control-binding/104338
Regards
Tinus