I gave it my best shot, but ended up coming to the same conclusion you did. A cylinder or plane is the best you'll get from SprutCAM at this point.
Your best bet is to do the projection in your CAD program and then let SprutCAM do a waterline cut on it. It will be tricky to get a good projection in CAD, too, but you can probably get something to work.
Good Luck, and if you figure something out, please let us know!
- Just Gary