You could make the freigter hull generate resources as was posted earlier, and then just make it a rule for your players that they must make the friegthers actually move around. There is no way to force them to move them in the game mechanics, But it would be fairly easy to verify. If you warp into an opponents system and he's got hundreds of freighters sitting around he's basically busted.
Get a group of players to buy into the idea and it could be quite cool actually.