I think the way it works with the legal issues is this: you can't use FPSC to create another game engine. I'm not sure how you would define that.
Technically, you're not creating a game engine, you're creating a map editor. You're letting the player design their own maps for use in your game and your game only, and I don't see anything wrong with that. You aren't letting them script, build new games, code, create models, etc. You're just letting them create maps to be played within your game.
However, the best thing to do is ask a member of TGC or hope one posts here and clears this up for us.
Avaric Entertainment
Team Leader/Lead Developer