One catch in the Marfa area is there's pretty much zero between towns out there, so if the towns are too far apart for you, you don't have much of a supply point.
There's a Fort Davis Cyclefest out there in the fall.
Bikin' Mike has had some group rides out there in the past, maybe Fort Worth Bicycle Association has too, come to think of it.
There's a "No Country For Old Men" race out there with 200 and 384 mile versions- put on by Dex Tooke.