Wasn't KLVI a Cowboys affiliate at one time? Certainly hasn't been since 2007 or so, but I thought I read that they were, which if so would be Houston's de facto Cowboys affiliate given the great signal. I believe NFL teams have some radius of exclusivity as far as radio coverage around their home market, but it can't be that much. Here in Ohio, the Browns have an affiliate in Dayton, only 55 miles from Cincinnati.
Not a Cowboys fan but I remember trying to pull the games in for the sake of variety years ago. Couldn't even get 1450 from Beaumont in the Clear Lake area, and WOAI was (and, I'm sure, still is) subject to a lot of interference daytime in that part of town.
As far as having a team in San Antonio, the Cowboys and Texans should do everything they can to block it. I just don't see it happening. Cowboys are way too popular and ingrained in that market.