There may be legal issues with it as well.
as long as this is considered fan art and not sold but given freely I couldn't see any legal issues at all. Honestly the X developers should be enjoy the free publicity to be considered a part of the "best of Sci Fi" mod. It seems they are improving the game along RTS lines with X-rebirth, but then it still will be a completely different game and not really a direct competitor creating "unfair competition" between sins and the X series.
Also the little factor in that they would sue a "little guy" gaining them little to no cash at all and you could just declare bankruptcy along the lines of the now bankruptcy friendly economy to get out of it. They would have to find a way to tie this to benefitting stardock and ironclad, which would be impossible since no connection aside from a piece of fan art could be made.
Don't be afraid of the legalities when trying to make something cool, as in the end you will have a lot of options.