It has always bugged us that there is no good way to hold the head door open. When using the head underway, it is much less claustrophobic to keep the door open, but then it bangs around and/or slams shut.
We came up with a very simple solution by attaching a small bungee to the bulkhead just inside the v-berth. When we want to hold the head door open, we just wrap the bungee around the handle. To keep it from banging against the other doorframe, we put a small rubber bumper on the corner of the head door.
It seems like such a minor thing, but this simple fix has made all the difference when using the head underway.