Thanks for your contributions.
@pentaxuser: just the contrary, there are multiple possibilities. The following systems I know of can be used on the Pentax 645N: Pentax 67, Exakta 66/Pentacon 6, Kiev 80/88 and Hasselblad.
These should also be possible as they use a flange focal distance that is greater than that of the Pentax 645 system:
Hasselblad 1600F clones
Hasselblad (200, 500 and 2000 series)
@adelorenzo: it is not the amount of blur but its quality I am worried about. Technically speaking, the amount of blur is proportional to the crop factor of the format, which means the Pentax 75/2,8 produces equivalent bokeh to an 50/1,7 on 35mm using the same perspective. There is not much lost compared to the 50/1.4 I use on 35mm.
@Mustafa: I've tested and used the following Pentacon 6 / Kiev 80 / Kiev 88 Lenses on my Pentax 645N via adaptors:
Mir-26 45mm f/3.5
Kaleinar-3 150mm f/2.8
Jupiter-36 250mm f/3.5
Telear-5 250mm f/5.6 MC
Tair-33 300mm f/4.5
I additionally tested them on an Nikon D5100 using live view, every lens throughout every aperture. The Kaleinar had a nice bokeh for a portrait lens, but contrast and sharpness of every lens left a lot to desire. This was visible in scans of 120 film even with resolution as low as 1200dpi. It might be, that the 80mm focal length lenses of the Pentacon Six system are better performer, and that might very well be the case for the Schneider Kreuznach lenses. But at least the russian made lenses did not convince me personally.