Lynn, I'm certainly not as smart as Judi but what I would do would be correct her for the back talk. I know you're using positive reinforcement only but her brain is kicking into her body and she's old enough to withstand a little correction ;)
And I agree about not using the play to get her to shut up. It's what Glenn was doing with Diablo (per the agility instructor's advice) when I first met him but I quickly saw that Diablo was then working GLENN whenever he wanted attention! So we got to the point where we're at now where he can't just be quiet when he needs to be because he learned in the past if he screamed enough, someone would come entertain him
A LOT of work is going into that boy to turn this around and it's not easy....
Work is work, she has no say about it. Give her a small pop with a stern vocal "EHH EHH!" and move on until you get what you want. At this point she might be so shocked that you corrected her that it might just be enough to end it forever...if you're lucky :p
Just my thoughts on the matter :)