If the MDA is too high for the pilot to remain at it until turning final and then make a normal descent for landing (while remaining within the lateral limits of protected airspace) then descent below MDA must necessarily occur on base leg or perhaps even late downwind.
Surely the solution to that situation is to divert - since descending below the circling MDA at any point other than within the protected area in the final approach splay, is potentially dangerous - unless the pilot can ensure legal terrain clearance for the category of aircraft.