Meh. And I almost got it to work, too.
In the end, GC relies very much on the fact that it has total control over the dimensions which are associated with a celestial body, there are several places where it keeps the dimension IDs, and it does a lot of checks to verify that the ID is really, really not from another mod. For this to work, the feature would indeed need to be added at least in the API. So, this remains a suggestion.
But, at least I managed to get custom solar systems to work. It reads a json file and creates stuff in the celestial map. Maybe that could be useful for something on it's own. If someone wants the code, just say so, and I will put it on Github. Otherwise I'll be too lazy.