At the same time, if an airplane can fly safely at MGW with an
underpowered engine and a dirty airframe, what prevents an airplane
maintained to a higher standard from flying slightly above MGW? I am
not suggesting that people try this, but except from a regulatory point
of view, what is the real difference between these two scenarios?

It depends on what the actual limiting factor is. If there is a
structural member that is the limiting factor, more engine power and a
cleaner airframe won't make a difference.

What I can see more easily is, for situations where (say) takeoff
aerodynamics is the first limiting factor, a graph of MGW vs density
altitude.

Jose
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