Samyang 8mm fisheye: rear-mounted filter?
If the lens does not protrude in back of the flange, you might be able to rig something. The mirror needs room, but there are examples of filters fitted across the flange. Astronomix makes such a clip-in filter, and the Sigma SD1 has a removable IR filter in that location. It might be simpler to rig something in a compact, as mirror fouling is not an issue.
http://www.astronomik.com/en/ for the clip-in kind.-- hide signature --
Interesting site and idea
However these are *not* ND filters.. and rather expensive also.
Emil Pozar, http://www.emilpozar.com/