As of right now, you can add it in the config file... but it does no good since it just defaults back to 1280x768! Big problem for me since my only gaming computer is on my TV which only natively supports 1080p or 720p and 1080p is just not playable on a 750M SLI setup. I'm getting maybe 20fps on average I guess. REALLY hoping they path the in... but I really can't understand why it's not there in the first place...
Conspiracy theory #1: they didn't put in 1280x720 so that more people with older cards hooked to TV's will go out and buy new ones thinking it's the only way to play it, they were paid off by nvidia and AMD!! ^_^ ;-) (stupid people, or just annoyed people with plenty of extra money!)
i am wholly in empathy with you indignation regarding the cost of constant pc hardware upgrades.
1. switch full screen mode to "1" or switch it to borderless window from the menu screen. ("real" full screen 720p doesn't work, you are correct.)
2. switch your desktop resolution to 720p
3. now use the config file to switch the game's resolution to 720p. as long as you don't mess with the in-game menus further, it should be fine.