Most professional video cameras have built in neutral density filters. That's one of the features that is hard to copy in a DSLR. There are a few Canon lenses that have a behind the lens gel filter holder.
Here's a link if you want to learn more about this feature. It isn't as convenient as the built in filters of a C100 or C300 but it is pretty easy to slip in a filter in the field compared to installing a VAF filter.
When I was a still photographer using 4x5 and 8x10 view cameras it was quite common to put gel filters behind the lens but this practices seems to have been largely forgotten. There are some huge filters designed to be used for wide angle lenses.

There is an interesting lens adapter called the
ND Throttle that has a built in variable neutral density filter but unfortunately it only works on mirrorless bodies. As far as the Magic Lantern enabled cameras not even the EOSM is supported.
Switching out the mirror for an ND filter is an interesting idea but it isn't a trivial hardware hack. The camera will not record with the mirror down.