I don't see this as much of a problem. Most builders wouldn't know
what they were looking at anyway. Some of them can't even drill a
round hole in the right spot :-)

If the manufacturer has preformed static load tests, with
documentation, and the design has been in service for a while I
wouldn't have any worries. I wouldn't buy/build one that didn't have
documented load tests available, or many years of proven service.

If you cock up something during building replace it. If you have the
ability to properly calculate and test the repair/modification you
shouldn't need the original calculations.

As for the EAA initiative I would be in favor of some kind of a formal
independent plans rating system. The old NASAD was one such group. It
could keep lots of people from wasting money on plans for things like
the FX ultralight and jet powered rotor cycles.

I do have the designers calculations for my Duster, but if/when I do
modify it I'll do my own calcualtions from scratch.
======================
Just my opinion
Leon McAtee