I am surprised that the Rokinon 8mm does not fully fit the 360 deg circle into an FF. The Sigma 8mm does. See this picture of mine:
http://www.astrobin.com/159762/
The flange back distance of 35mm lenses are much shorter than that of 645 lenses. So there is no convenient way (or entirely no way) to fit the lens to 645 and expect it to work. To adopt a 35mm lens to MF, you need to use a camera back rather than the full camera body. This is what I got after I fit a Canon 15mm fisheye to Mamiya 69 film back:
http://www.astrobin.com/77503/
I suppose you can do the same to an MF digital back, but I believe you will go through lots of troubles that are probably not worthwhile. Just getting a Sigma is much easier.
Alternatively, you can take a picture with your Rokinon 8mm, rotate the camera/lens by 90 deg, and take another picture. You should be able to get a full circe after blending them together.
Cheers,
Wei-Hao