Yup, my girlfriend has braces at the moment (got them for me) and will wear headgear for me. Can kiss her with it, have sex with her with it, whatever.
I will say it over and over again, when you find someone who loves you enough she will do anything to make you happy! Don't be scared, just ask! (Or get your braces pictures found by your girlfriend like me lol)
Even if when you have a girlfriend she dosn't want to have real braces she can have fake ones and headgear and it's pretty similar. You just gotta say it's something you really wanna do! Try making her fantasies come true, ask her if there's anything she would like, maybe that she's been too embarassed to tell someone else about. Then she's more likely to do something for you she may at first think is weird. Chances are she'll get used to it and start to enjoy it, it's a turn on for girls to feel attractive to their man.