So is there any way at all when making maps via the in game map designer, or possibly using forge, to prevent the random extra star containing the occupiable planet?
I was hoping to make a map that focuses on the occupation victory condition. Yet despite setting the occupied planet to spawn in a certain place, and even looking like it works in the map preview, i always still get the random extra star with the planet (which overrides the one i placed for the objective) when i start an actual game.
Anyone have a fix / way around this?
Nope. 
Sadly the extra solar system is hard coded to appear if Occupation victory is turned on. Even worse, only the occupation planet in that system counts for the victory. You can put extra occupation planets, but no matter how long you own them you will never get the victory.
The only thing you could do I think is either have all players start right next to the star (and thus possibly rush to the occupation star), or have the only connections to the main star be wormholes, which you can then put around the map to make whatever challenge to get to the occupied planet you'd like.
