We had a private party at Piccolo Forno a few years back and it worked out very well. At the time at least it was BYOB, not sure if it still is or not. We didn't think the price was bad at all considering how much food they gave us. They were also very easy to deal with and custom tailored the menu for us, didn't have to pick from a pre-planned package. I THINK it was in the high teen's per head, but I'm not certain.
I don't know the South Hills well, but I think I can make a few suggestions in the Pittsburgh area. You could try Piccolo Forno in Lawrenceville. They do have private parties. Cafe Roma in Bloomfield is also willing to work with you for private parties. Try Cafe Zinho (Shadyside). They are so good and it's a really nice space. I've been told to check out Cafe du Jour (Southside). That might be a good place as well. Ok - and one more place to look into - Pizzutti in Shadyside. I hope these help.
I was at La Tavola (Italian - Mt Washington - BYO) last Saturday night and they appear to have a decent sized (12-15 ppl?) semi-seperate room for events. Their website says the cater and do events....my dinner experience there was good, not fantastic but it might be worth a look....not overly fancy, but lights are dimmed and there are white tablecloths....nice enough....good reviews on CH as well
I cannot think of another BYO that is really large enough to host a rehearsal dinner unless you are pretty much renting the whole restaurant...I speak more for the city though. Perhaps someone knows of more places in the South Hills?