Stuart. Not a bad way to do it, only I have all my power coming on at once from a button on the front of the machine.
This turns everything on at the same time. Because of this, the relay switch I have works well.
Another way you could do it, and it sounds like something you have implemented, would be to use a charge pump. That way until mach and the ESS are communicating the charge pump relay is open, then the relay could power the rest of the machine.