Hi Courage, I don't do AA but I do know the city fairly well after being here for a few years.
I would advise against a meeting in the Tenderloin. It's no longer as dangerous as it once was, I walk through there at night sometimes. But a meeting there will probably have a higher percentage of mentally unstable people. No nice way to say it, this city does have a very large population of that demographic and the Tenderloin and Civic Center will be meetings you'll (probably?) want to avoid.
I know that Pacific Heights has a good reputation on Yelp (an AA meeting with a Yelp page? I dunno but they have one) and isn't too too far from Union Square.