"My idea of evidence is things like the ultimate breaking strength of a wing taken to destruction (which you must do for EASA certification)"

It is up to GP to negotiate with DULV (Germany), but ultimate load test will have to be performed and DULV will probably ask to go to rupture since it is a brand new design. DULV also has to witness this type tests.
I was skeptical about UL for a long time until I took the time to dig into the different national regulations and realize most countries had very high standards.
if you can read German, here is a link to LTF-UL which defines requirements for approval.
https://www.dulv.de/sites/default/fi...V%201.22_0.pdf
There is an older version in English available from the net: https://drive.google.com/file/d/0BzP...ew?usp=sharing