1) The only way I would use Batgirl was if Barbara was dressed as "Bat-Girl" at a costume party... if she happens to help Batman catch some armed party-crashers or something, sure, why not? Even little girls can dress up as their favorite urban legend and yell "watch out!"
I would not, under any circumstances let some untrained girl wander around the rooftops in body armor. Period.
2) If I were to use Barbara Gordon as a supporting cast member I would use her A) As she was in the Kililng Joke, and transfer that into a character development issue for Captain/Commissioner Gordon. B) Criple her and have her drop tips to Batman via computer as an 'Oracle.'
3) If I had to cast Batgirl, I would definitely cast a teenage role, especially since we've seen her as a little girl in Begins. I'd probably cast someone like Taylor Dooley or some other bright faced kid who can pull the downtrodden look off and speak intelligently.
4) In a perfect world, she'd have a scene with Batman at the party a couple small scenes with her father, a scene getting shot and stuff, and a couple scenes doing the computer thing... there's all the Batgirl anybody needs, and that's not too much Batgirl for anybody, not even Nolan.
