As a contrary data point, my (Canadian Marsh/Lloyds)
insurance includes an explicit requirement to stay within
W&B limits to retain coverage.

Which may or may not be enforcable, depending on state laws. It is
true, thought, that some carrier demand in-envelope operation, and that
others required the airworthiness cert to be in full force and effect,
and then argue that over-weight operation voids said cert. Again,
whether this latter approach would work would depend on the state.
Avemco, though, specifically say that you can be over-weight and
they'll still pay. And no-one has yet produce an example of *any*
company failing to pay as a result of an aircraft being over-weight....