I looked at this problem with the Lee filters people, I was looking at getting an ND for a Sigma 14mm, and we decided that it really wasn't a practical thing to try and do, although if anyone else can come up with suggestions, we'll look at it again.
The real problem as I'm sure you can imagine is the fact that the front element would almost have to touch the filter, and even then, to avoid the holder being seen by the lens, it would have to be pretty massive, and the filters would probably have to be specially made- a pretty expensive exercise for what we saw as really a small market!
Hope this makes sense!
George