Generally, Half Arabs show in the same halter stance as purebred Arabians. I have a registered Half Arabian who I've shown halter in rated breed shows, unrated breed shows, and open shows. She is an Arab-Quarter cross, but looks full Arab so even in open shows standing in the Arabian style stance was preferred for her. At breed shows, whether rated or not, I've never seen a half Arabian stand in any stance other than the same style the purebreds stand in no matter what their cross is. I'd imagine at some open shows though, if your mare doesn't look very Arab, depending on what classes you show in, you may want to stand her in a squared format rather than the traditional Arab style.
The Arabs generally stand with their front feet square, 1 hind foot square, and the other hind foot slightly back more. I want to say that the right hind foot is the one that stays square and the left hind is the one set further back, but I could be wrong. It's been a few years since my mare showed halter! If you look at halter photos on any of the big Arab breeders websites though, you will see the correct stance.
Hopefully this makes sense :)