[mst]

I guess it's a question of available space in the camera body. Plus, it would be an additional layer of glas wioth at least some amount of refraction, so it'd be part of the whole optical system. That's why I guess it would have to be two filters, a clean one and the ND one in order to keep the whole optical system at the same level (similar to high-end tele lenses, that allow drop-in filters and need to have a clean filter in there).

 

In terms of dust, I'd be afraid that it just moves the dust issue to a different layer. However, a protective glass cover is what Sigma uses in their own DSLRs, so in principal the idea seems to work.