The ND filters are physical elements in the lens. On the G1X you can see and hear it engage, and the X100 should be the same since Fuji's literature also mentions it being part of the lens. The point being, ND filters need to be built-in to the lens not the body. If software ND filters were possible we'd just have cameras with ISO values of 100, 50, 25 available instead.

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Nic
E-M5, GH1
Flickr Photostream
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Video cams have the ND filters in the body, and it does work on larger bodies, as the EOS C300 and C500 both have them.

I guess available real estate becomes an issue when mounted in the body. An in-lens ND filter is quite small, but mounted closer to the sensor it would have to be basically the size of the sensor itself.

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Nic
E-M5, GH1
Flickr Photostream
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