A 'pop-up' filter drawer would help speed things up switching the ND filter in and out - that would not be too difficult to make for the tele lenses with filter drawer.
Would also make it for the Polfilter as well,,, but a dedicated version might be needed.
Sort of the same/similar solution as the new 180-400mm that pop in the TC in and out,,, or like the really old dias projektors did with the slides.
All the filter-wheel designs don't really work here i think,,,