Yes you can change resolution outside the game.
Go into your Start Menu and then under GOG and Omnikron there is a link to start the game and also one called Settings, use settings to select your resolution.
It opens in a small window and you can select graphics driver and resolution from 2 separate dropdown menus
Well thanks to Iain I'm one step step closer, however most resolution choices in the settings don't stick, and the game defaults back to 1440x900 no matter what I set it to, the only resolutions I can change in the settings dropdown that stick are any resolutions that are smaller then 1440 wide.
Any resolution set higher then 1440 wide in or out of game does not stay and always will default back to 1440. Also being able to look at the list of available resolutions, it seems the only one listed that won't work is my native resolution of 2560x1600. Which may lend some credence to what timeslip said.
So unless not doing something I'm supposed to (beside confirming the resolution change when I make it) Then I'm pretty much stuck having to change the resolution 18-21 times every time I start the game.