Fair Use rights are based upon the level of infringement as balanced against four equally ranked categories. Those four are-
1. Purpose of the Use (Better if not for profit, etc.)
2. Nature of the Copyrighted work (The more fact-oriented the work you copied from, the better it is for you)
3. Amount and Substantiality of the Copied Work (In the case of the mod I am afraid that's a lot- both quality and quantity)
4. The Court's belief of the effects this copying will have (or has had) on the market for that good.
You could come up with all kinds of arguments both for and against creating freely-distributed fan-made mods for GalCiv and SEIV based on the above categories. Your best bet would be to examine various and sundry cases in this area in an attempt to find loose analogues. I personally think it sounds like a bit of a crap shoot either way, but what do I know, I'm just a law student!