i haven't got anything detailed to post right now, but when we start our next cowboys campaign (its never long between campaigns) i'm planning on using a re-working of the Planetary Empires rules for 40K along side the standard campaign rules for Legends of the Old West. the PE rules are incredibly basic and could easily be adapted to most systems. i don't know TRWNN, but i can't see why it wouldn't work.
a lot of people don't like their simplicity, but that's exactly what i like about them. they aren't a game in themselves, they just make a map based campaigns quick and easy to deal with.