I was just thinking to myself.....with the shallow flange depth on this
camera, would it be possible for a lens adaptor to be built with a
built in ND filter wheel? Not 100% sure on the mechanics behind this,
but it seems like something an enterprising third party could build...
Just random thoughts on a Friday.

Or maybe a drop-in filter slot like you have on large tele lenses.

or would this throw off the back focus at all? -- probably not much of an issue for primes, where you just would be a little off on the scale, but bad for parfocal zooms.
Of course such an adapter would have to be rather large, to be able to hold the (rotating turret of) filters in stand-by, out of the light path but nearby.

It'll be a problem for wide angle lenses... you have to design the mount factoring in the ND filter and an optical flat when you don't need ND.