Does anyone know of a public range in south Texas with 600 to 1000 yard capabilities? Or one with open membership.

I list three ranges in TX that shoot to 1000 yards - see http://www.elr-resources.com/page8.html. I did not drill down to determine location and the other things you are looking for.

The NBRSA is running matches out to 1000 yards in TX but such matches are limited to .40 caliber or smaller. That location is one of the three (Emert). They also compete at 600 at the Pine Valley Benchrest Range - Southwest of Huntsville Texas about 8 miles.