Cool! That sort of worked!!
I downloaded the utility and launched the autohotkey script exactly like that "Lazlo" guy said, except I mapped it so that the mouse would only invert when pressing Alt-Y. I compiled it and tested it out and everything worked flawlessly... When I pressed Alt-Y, the mouse Y axis inverted as expected and turned off when I pressed Alt-Z. The pointer would not get stuck in the edges or anything.
In the game however, things didn't go as smoothly. I fired up SH5 with the script running in the background, and when I pressed Alt-Y, the Y axis did indeed invert, but the POV kept getting stuck everytime looked up or down at too high of an angle
It's a start, perhaps with a little tweaking this could become a great little work around until ubisoft implements a real fix.
I fear that I am a bit over my head with this sort of thing. Would any of you smart people out there care to give this a shot and see if they make a custom invert Y axis script for SH5? You'd be my hero!