Thank you all for the suggestions - I will try her in a mullen and see what she thinks. I don’t think she has a low palette (I asked my vet to look at her) but she is an opinionated mare. I like her in a curb chain for those wild hunts but she gets a bit snarky about it on slow days.
I’ve never seen this bit in person - is there some flex in the mouthpiece between the joints, just less than a snaffle?