Good luck! I don't know of anyone currently digging in that area, but the Sanborn Maps for San Antonio should be online and every house in the older sections would have had a privy...that's probably your best bet if you can get some permissions. There are some absolutely incredible San Antonio bottles out there, so good luck!
Hey! Im in SA, digging here is super hard. I havent been lucky in finding any places TO dig in SA. I've asked a lot of houses in the Lavaca area, but have been turned down. The backyards are also super built up, and most of the 'old' houses only had an outhouse for 5-10 years before they had plumbing. Most things older than that (ie 1880s, etc) are virtually buried under the metropolis.