Hi Nick, I don't know if is in dvelopment a such panohead but I think worth posting here some considerations, kind of wish list, maybe other to contribute as well.
I think a panohead ring lens type but to allow arbitrarry and with detent plungers tilting preserving the NPP and arbitrary and dettent plunger rolling (i.e.) to allow rolling quick from portrait to landscape but with dettent plunger for 0¬? 30¬? 60¬? 90¬? positions.
Personally I hope for a camera with round sensor but I saw that you are lessen way better than the camera manufacturers and the rolling on the fly will be very good way to use the 8 .. 10 mm fish eyes on crop cameras and even on FF (zoom fish eyes) according with the scene or technique.