Guys,
As far as I have always been told, if you are circling, the circling MDA is the minimum altitude for circling until in a position in the circuit to carry out a normal descent using normal rates of descent for the aircraft type to the landing threshold.
The circling MDA is designed to give you the required Obstacle clearance of 300' Cat A & B or 400' Cat C&D anywhere within the prescribed circling area.
You cannot circle at an altitude of your choosing that you think will keep you 300 or 400' above all obstacles based on where you think the critical obstacle(s) are.
If you cannot remain clear of cloud with the required visibility as per the chart and keep the runway environment in sight whilst at the circling MDA then the weather is below the circling minimums, and you shouldn't be there, ie you should have executed the MAProc.
Also I was told, by examiners both here in Aust and in the US that if because of the aircraft's design you cannot see the runway and keep it in sight ,for eg when trying to circle to the right, because of the wing, engine nacelle, viewing angles thru cockpit windows or whatever then you should not be doing that either.