Contact game news publishers and strongly advertise single-player, multi-player, full mod support and that the game will continue to get updates. I recommend making open-source a main selling point only to publications/communities that care, because to a lot of people(who don't care or are ignorant) open-source means low-quality or community built. Then there's reddit(r/megaglest and r/glest are taken by someone who hasn't replied to me), facebook groups, twitter, etc.
It would help also to come up with a set release date.