I wouldn't use any contraptions on her, especially a neck stretcher, as that teaches the rider to be dependent upon it and the horse to learn a false frame and give her a neck that is more developed below than on top. Plus, you can't take your horse into a hunter class in a neck stetcher. You can get your mare to stretch her head down, but please remember the conformation of her breeding gives her a higher set neck and a more upright shoulder.
I ride with a woman who has a Saddlebred who was giraffe neck high and would draw away from your hands with the lightest contact. She's worked very hard and has taken it very slowly, but she now shows him in 1st Level Dressage and schools 2nd. Both this woman and I ride with the same trainer, and he encourages (for this kind of horse who picks their head up and draws away) a long and low frame to start out with before compressing the horse any. It takes a lot of time and patience, but it can be done.
If you ride with a trainer, ask for his/her opinion on how to go about this. If you don't have a trainer, it might be a good idea to find one who can help you.