There are c-mount to Canon EOS lens adapters. Could take this project to the next level!
It's nice for things like astronomy (your nice 200mm f/2.8 is now 1000mm+) or bird/nature watching, but it's highly impractical for anything but specialist portraiture or walk around use.
Most of the c mount lenses available are pretty poor, and getting focus right with anything not way stopped down (especially without a large display) can be super tricky.