Interesting idea.
Though as far as Clairmont's Alexa BTL Filter, I worry about the strength of the magnets. Magnets close to sensitive electronics, especially the sensor, sounds dangerous to me. I'd consider a different method.
After thinking about it, I wonder if one could apply a behind the lens variable ND filter to the inside of an adapter to make what the OP wants. Depending on the adapter size, B&H states they sell 37mm, 43mm and 46mm Variable ND Filters. I plan to look into this.
It looks like you're new here. If you want to get involved, click one of these buttons!