Ceegap: I also want this otherwise the game is unplayable for me. (30 years of muscle memory with joysticks)
I see in the patch notes on Steam ( I bought it here though) that it'll be in the next patch. Yay!
Inverted mouse is now available in build 30 which is live here on GOG.
so if you are playing an old save, you will need to manually insert 1 line into you player file. otherwise, your selection will NOT be saved... and you will have to select this option with every load of the game.
new games will be fine.