Haven't been there in years, but there a lots of campsites and campgrounds in the Big Bend area. Terlingua isn't far. Check with the national park and the state park authorities.
Yeah that's my next bet is to simply camp in the National Park which IIRC you can camp anywhere as long as it's not marked "No camping". I'm just trying to find a place to camp closer to Lajitas since that is where the MSTA group is seting up shop.
