My daughter has had a couple of scans to monitor her hips because she has talipes and it can be linked to hip dysplasia. They had a special wedge thing for her to lie in to keep her in position and then they just gently hold one leg while scanning. She doesn't need to stay perfectly still for ages because they take still pictures on the ultrasound then measure those.
Sometimes the hip corrects itself, my daughter's has. She had a scan in December which showed one hip being slightly immature, as they put it. It wasn't quite in the joint, but it wasn't fully dislocated either. They said it would probably resolve itself in time and that she wasn't in pain because of it. She was rescanned this week and it's okay now. They recommended wearing double nappies to help her lie with her legs apart. My girl likes lying in the frog leg position anyway (though she can't now because of the leg cast for the talipes, poor thing!) so that helps. If you use reusable nappies that's good because of the extra bulk between the legs, so you shouldn't need to double up on them.
I was concerned about it because with her already having a toe-to-hip plaster cast on for the talipes, it would have been so awkward to have the parvlik harness on as well but fortunately she doesn't need the harness.
There's an organisation called STEPS who have a lot of advice and they have a fairly active forum where you can speak to other parents, if you like.