Is it possible to restrict the maps available for campaign mode? For example, I downloaded Gotcha's map pack, and I want to limit the maps used to only the space maps and a few specific snow maps. How would I go about doing this?

If I were you I'd just disable the non-space and snow maps by removing .rte from the appropriate folder names.

If you really want to disable specific maps in an .rte you'll need to edit Index.ini and Scenes.ini in each .rte, putting "//" without quotes in front of the line you want to disable.