Why hello there! I'm Rob and I run RB-San Francisco. Right now we run out of 1 venue, but it's constantly recurring and we partner with some people that hold occasional events in Oakland. To answer your questions:
1) Personally? Not very far. Our own events are in the city, about 4 miles away from where I live. Even going to Oakland, it's only 16 miles over. I know that some of our regulars come from Berkeley area (about 20 miles), most are here in the city, some come from the South Bay/San Jose area (about 50 miles). One of the big differences is that I don't own a car; in SF you hardly ever need one. Anything in the city is less than about 5 miles away from anywhere else in the city, so most people dont need/have to go that far.
2) Why did the thread slack off? Most of it had to do with the RBBN program going away. I'd imagine that another big part of it is that the people running RBNights have been doing it long enough now that there's no real questions left about it. There's also all the area-specific threads that takes out most of the 'general discussion' aspect. There's even another (newer) "How do I start a Rock Band Tournament" thread going on for more questions.
3) I have some of the promo-art that came out, but some of them are too big for proper attachments. I'll see if I can put them on megaupload or something later.
4) Costumes? If you have them readily available and that's going to be part of the focus for events, then it wouldn't hurt. I'd say that, very broadly speaking, it isn't necessary, but would be a nice extra touch.