If she's showing in the hunters, anything but a regular noseband won't be allowed - dropped, flash, figure 8, etc.
If you wanted a little bit of extra leverage for a big strong horse, pelhams are acceptable in the hunter ring. I think french link pelhams are pretty easy to find if you wanted to keep the same mouthpiece. A friend of mine has a 17hh Trakehner x TB eventer who can be quite strong and hard to stop (I can say that from personal experience!), but she does quite well with him in a jointed rubber pelham, just like this one: http://img.smartpak.com/product/kors...umb_pelham.jpg
Hope that helps some! :)