While not ideal, you can run the game through Steam, enable Xbox (or whatever controller you're using) configuration support in the controller options, then in the controller settings for the game there's options to invert analog axes.
This does not work for keyboard and mouse :(
But the mouse is not inverted.
However if you wish to play with inverted mouse, I believe there's a number of mouse inverter programs out there. I found this one, for example: https://superuser.com/questions/406502/how-can-i-reverse-mouse-movement-x-y-axis-system-wide-win-7-x64
EDIT: Sorry to say, I tried it and it only works on the desktop. Guess you'll have to keep googling for a solution. Presumably there's even a global setting for some Unity game config file to invert mouse controls.