Just had a good look at that discussion... IMO if the overload trip characteristics are known (in Europe and all other parts of the world that use DIN rail equipment the curves are standardised) testing the overload trip would be easy.
For example, most breakers are supposed to trip within 1 hour at 1.45x the nominal current, so testing is just a matter of connecting a known overload and measuring trip time. An overload trip is also considered less likely to damage the breaker than a short circuit trip which involves much higher currents. If done by an individual that doesn't equal an official listing, but it should be well possible to test breakers with that method under official lab conditions.
I once had a breaker with an unknown (ancient) trip curve and used that testing method to at least guesstimate the time trip curve. It was a 10 amp breaker, so I started with 12amps which caused it to trip after about 30 minutes. Then I went to 16 amps which tripped it in less than 2 minutes. Certainly not scientific (particularly since I only had one breaker to test) but more than a good guess.