My neighbour has Shettys and used a dually on her late cut gelding. He's probably just tall enough to be a standard though (I don't know his official height).
I would just buy some yacht rope and make a richard maxwell style halter. I've made a couple from pictures (when I couldn't find my original in the tack room) and they are easy to make ?
My minis have the purple Duallys. They look a little 'busy' on their tiny faces but fit well and do the job!