Can you imagine how grumpy she would be if she waited to deal with this as a teen? I know that brace has to be the pits, but it's for her future life and that's pretty darn important.
Be understanding about her grumpiness, but be FIRM in discipline and your expectations of what she does and doesn't get away with. She will be a holy terror as a teen if you let her boss you around. When she throws a fit, send her to her room. NEVER let her get her way or she will act even worse.
There is a book that you should order from Amazon (or it might be at Barnes and Noble). It's called "The Berenstain Bears Get the Gimmees". The little girl bear starts having tantrums in the store when she doesn't get her way. Just looking at the pictures of her having a tantrum helped my kids to think about how THEY looked when they had meltdowns. Maybe it will help your daughter too.
If you consistently send her to her room and make her stay there until she stops her antics and never give in to her demands, she will finally stop this stuff. Meanwhile, no TV, no computer, no electronics in her room. You can do it, Mama...
Sending you strength~
Dawn