In principle this has always been possible because FAI and IGC (rightly)
cannot control criteria for events and flights for which the rules are
made by the National body.

This is true, but it does not apply to your exampe, below:

For example, one of Jim Payne's USA 100 km triangle records was for a
time at a faster speed than the world record. This was because of
different criteria on the shape of triangles.

In fact, the criteria were identical when the flight took place.

The IGC criteria changed after the flight, specifically for the
purpose of disapproving it.

It has always been the intention and practice of the USA to use
international rules for the homologation of national records.

-Pat