Sorry for the delay, Scho. I had no access for a couple months. I just looked at your Iris images with the V 180 adapted to your 100S, and "impressive" is the best one-word summation. Beautiful images, and spectacular quality. Thanks! I was thinking of using my V Tele-Superachromat CFE 350mm, and its matched 1.4 teleconverter, on a 100S. Do you think an adapter could handle the weight of that behemoth?
I use the 350 Tele-Superachromat + APO 1.4XE teleconverter on my Cambo + Phase One IQ4 150, which is in essence a larger version of the sensor in your GFX100S. Image quality is superb. Assuming you use the lens foot to mount to the tripod, you should have no problem hanging the camera body off the converter - I do so routinely with my Hasselblad X1D and in a pinch with the much heavier Cambo/Phase combination. However I would never dream of mounting the entire set-up via the camera's base plate, with or without the teleconverter there would be too much weight hanging off the front end.
If you already have the better Hasselblad V system lenses (IMHO: 40 IF, 100, 120 Makro-Planar, 180, 250 SA, 350 SA), trying them out on your Fuji with an adapter is a no-brainer. I've posted a number of images on the Fun with MF Images thread that you can browse through for examples.
Hope this helps!
John