Colour slide fillm are only colour slides if you develop it in E-6. If you crossprocess it in C-41 you get negatives. So why not crossprocess the C-41 B&W film into E-6? You'll end up with B&W slides I think.
Or why not consider any of the B&W slide processes which use B&W film?
Supposedly you can process colour film in B&W developer to get yourself bad B&W negatives but I don't think that's your goal.