If they are trying to board via rows for Y, it is not a trial but another attempt at it. I remember seeing this method at various outports such as KIX and BKK(?). I am pretty sure they tried doing so at HKG, even announcing that they would first board those with priority boarding and then those at the back of the plane.

The success rate varies but I contend that it depends more on the behaviour of passengers. At KIX, I remember the kids could not care less about announcements and would rush to form a line once the boarding announcement was made. It was always a bit chaotic there.