So... I've been messing around with making a new add on for my controller system and this is it in its embryonic (cardboard and electrical taped chassis) form.
It has 3 dedicated keys on the controller for incrementing and decrementing in stops (it goes to about 95% total blackout) and a clear function. Though this is not a full clear as there is about a half stop of ND at the lowest position.
The level of reduction is actually fully variable but for the implementation I'm after