You might try alternative ways of setting up the panohead. It's certainly true that many real-world rectlinear lenses don't have an entrance pupil located at a single point. Michel Thoby wrote a paper pointing this out some time ago at: http://michel.thoby.free.fr/Fisheye_...al_lenses.html . Whether your particular lens has issues in this area I cannot say, but we cope well enough with fisheye lenses, so it is probably possible to get a good stitch with it.
Thanks for the informative reply. I'm still learning and testing all this stuff and it seems that depending on the stitching software you use, the parallax can be corrected to a certain extent. Although I'm not absolute if stitching software is the answer to these problems, it still offers a workaround for these types of situations