Looks like you’re right. I just tried to put a multichannel effect on a track but it only give me a stereo option. That’s extremely frustrating. I’m not sure how to proceed here in this case. If I could manage multichannel plugins I would simply build Cabbage and run it through Cubase with the debugger to find out what’s going on. Without a copy of Cubase I can’t see this issue being resolved unless you happen to know your way around C++? In which case you could have a go at debugging the issue yourself.
Steinberg frustrates me so much as a developer 
I can’t justify the cost of a license just to test a few plugins.