Hi Lars,
Very interesting. I also have the Pentax 6x7 165mm f2.8 lens. I have used it on a Pentax K-5 (APSC) DSLR. I found that I needed to stop it down to f5.6 - f6 to get rid of chromatic aberration (violet halos around brights stars).
My two best pics with this lens so far are probably these:
https://www.astrobin.com/278971/?nc=user
https://www.astrobin.com/264019/B/?nc=user
I would be interested to see your results.