Only Round Texel and one other race actually checks if you have insurance.
Not sure what the legal reason is but it may be part of the conditions from the organising regatta's insurance policy.
I
could mock up something that
looked official quite easily on my PC; if inspected(and approved), would this transfer liability to the organisation?
I believe that that would technically be considered fraud on your part.
Agreed, but only if spotted by a hassled person at check in for an event.
My point is that checking the bit of paper adds no value to the process and MAY transfer SOME of the liability.