Well your picture is somewhat of a torture test for lenses, especially older ones. The ultra-creamy bokeh is a more recent phenomenon, and I haven't seen much in the medium format range that performs like (for example) the new Sigma 50/1.4 or 85/1.4.
You can look up the Pentax 67 105/2.4 lens to see if that is acceptable given your focal length requirement. (I personally love the rendering of that lens.) Another lens which will likely produce smoother bokeh than the 75/2.8 is the Pentax 120/4 macro, again outside your focal length. I have both the 75/2.8 and 120/4 and quite frankly carry around the 45/2.8 and 120/4 almost exclusively.
Below is a photo with the Pentax 67 90mm f/2.8 LS lens (on a P67ii camera) which I doubt would be starkly different than the 75/2.8 but you can decide for yourself.
I have other 645N samples which you can browse if you want to: http://blog.seancarpenter.us/tag/pentax-645n/ I have the 45/2.8, 75/2.8, and 120/4 macro lenses for the 645; and the 55/4, 90/2.8 LS, and 135/4 macro in 6x7 format.