@ Nick Hope
School photographers use background rollers top and bottom to rapidly change backgrounds. A miniature version with gel NDs conceivably could be rigged in a housing. I designed an Arri M housing with a swing in filter years ago.

My this is an impressive tool! I think about all those years of shooting with ECO at 25 ISO.
Ralph
Another feature of that housing was a dual waterproof control. A 1/4" SS shaft with o-rings inside a 1/4" ID SS tube thru a 5/16" gland. The outer tube had an arm with a rubber wheel and mini chain drive to the inner shaft and could adjust lens aperture and focus. A similar device could easily swing in a Singe-Ray Variable ND filter and adjust it. There are also Micro Motors made by a German company (Graupner, I believe, that are only 10-12 mm in diameter with gearboxes.) I built a zoom and focus control for a 10-100 Zeiss zoom with them. Instead of the dual shaft above you could have a swing in filter with a motor driven density change.