Higher than 800x600

There is, you basically edit vegastrike.config I have mine set to 2560x1600 The engine will do any custom res. The only problem being some art assets aren't high enough resolution so they will be stretched. The ingame will render at any resolution you feed it. Try 320x240 for some old school playing.
I already thought about a widescreen version with a better resolution (baseart too) but honestly the project has bigger issues engine wise we are currently trying to solve.