Ok, I tried some things.
The most promising lead was this post by Dale RhinoCommon Brep split by plane
I did like it was written there, I get a PlanarSurface, when I draw it, it is straight throug my object etc. But Brep.Split simply continues returning 0 objects. This is slowly a bit unnerving.
Effectively, I just need the functionality of the BooleanSplit Comman in the Interface. I also tried RunScrip to use the BooleanSplit directly, but it also just returnes with false…