On the subject of not being established at 500' on the approach:
Difference between levelling off at MDA and circling is that circling is a purely visual maneuver. It's just like part of a traffic pattern. Since circlings are very rare anyway, I wonder how many of you would do a circling 50 feet below cloudbase. Wouldn't think so.
And it's common sense to make a decision on maneuvering your 747 at 400 feet above the ground.
By the way, in our company, circling is done mostly on autopilot.