I put my 5D in my 30/60 housing with the old mount and it 'kind of' fit.
Will need to modify the mount, or build a new one in order for it to slide and lock in.
You loose all of the back controls since they are moved to the far left on the 5D.
With minor modification you can control both wheels. Shutter release needs no modification. I think if you don't mind limited functionality underwater it could work.
I have never modified anything on the S&S so wouldn't feel comfortable adding any buttons or knobs.
I've been playing with the 5D on land. Wonderful camera (especially since I had a D60 before).
My sigma 12-24 is now truly a wide angle.