The question is in the subject title...
Have had some three decades ago a ring to fix Zeiss Hasselblad lenses on Contax. Very hard to find today.
I think that a reverse ring for XCD lenses should be a great thing, even if the AF should be lost... ???
;D 8)
Besides AF, you also lose the shutter and flash sync with it.
Cambo has Hasselblad X adapters for one of their technical cameras. They allow you to mount any lens in any orientation at any distance. But the camera is from the early days, when the X1D and 907 50c were current. I don't know how well it works with the X2D (re: lack of trigger input).
For higher magnification AND flash, you shouldn't discard options that start with a native lens on the camera. For example, diopters. Adding a reversed lens in front of a normal lens. Mounting an infinity-corrected microscope objective in front of a normal lens (using it as a tube lens). Etc. AF and IBIS may or may not work, but shutter and flash will work for sure.
Thank you for your very precise and wise answer :)