Actually, screenshots demonstrated it would add more to the sides. Switch between these:
http://widescreengamingforum.com/f/u/imagecache/node-gallery-display/contrib/dr/25/ingame_16x10.jpg
http://widescreengamingforum.com/f/u/imagecache/node-gallery-display/contrib/dr/25/ingame_16x9.jpg
Only, I think the trick is, when you use 16:9, that you have to pull back the FOV, to compensate for anything lost.
Like you said, the FOV can easily be changed.
Maybe I could do that indeed, to just make up for the width, but then the 2D-elements are still stretched vertically, which looks quite bad.
It does work when I change the desktop resolution to 1600x900, then the game will simply also show letterboxed like the desktop already would.
But I'd have to switch to that and only when playing the game, and it's quite a hassle to do that every time.
I can not understand why every other game simply rescales into the right aspect-ratio and only this game doesn't.
It's not my monitor doing it, it doesn't have the capability to rescale, I think it's from just before the time that monitors got that feature.
And it's especially strange since this game would specifically support only 4:3 and 16:9, yet it won't letterbox (or even force letterbox like 'Assassin's Creed').
I also don't understand that it's said it only supports 4:3 or 16:9, maybe because of the 2D-elements, but it does allow for 16:10-resolutions.
The only thing that seems to be contradictory is that it's about the only game that doesn't let a 16:9-resolution be letterboxed.
That goes against the use of 16:9-resolutions AND the fact that they included optional 16:10-resolutions in the menu.