Many thanks @nougad for all your reverse engineering! Out of curiosity: Are the KIX 300 APIs the same as the ones of the KLF 200?
A link to the fully documented KLF 200 APIs can be found in the docs of the Velux binding. On that note: Based on my understanding of the API docs, KLF 200 supports more than 5 devices (up to 200 if I get it right).
I could imagine that Velux is using parts of the KLF 200 code base for the KIX 300 (this would mean that the existing integration could be amended without too much effort). And given that Velux documented the KLF 200 APIs I could further imagine that they would eventually do the same thing for the KIX 300.
I will certainly follow this discussion with high interest! I just installed a Somfy Connexoon to control my two Velux roller shutters (the rest of my apartment is KNX), just to find out that things are controlled through the Somfy cloud (which I don’t really like). Setting the Connexoon up with my OpenHAB Raspberry worked like a charm, but I will eventually replace it with a solution with more privacy.