I've searched all over the web, and this old thread has the only reference to this hidden easter egg.

The easter eggs main thread (mbx.streetofeyes.com/index.php/topic,2564.0.html) has a reference too, but without any clue on where to find this cutscene.

Since I've looked everywhere within the game without luck, I've explored the mod files too, to find what follows:

when speaking to an "hall guard" he says, instead of the usual greetings, that it's midnight, and that at the midnight an "unusual accident" happened there, mentioning that a "man, a dwarf and an elf"... And the dialogue concludes, probably interrupted by the cutscene.

Since we all know who these characters are, I've followed their path with this map:

http://lotrproject.com/map/#zoom=3&lat=-1315.5&lon=1500&layers=B

But I visited at the right time all the cities where those characters should have stayed, and I talked to all the guards, inside and outside the castles, and none triggered the cutscene.

By the way, the original clue given in this thread should be right, as in Erech actually happened something, and Erech was the first place I visited, but without luck.

I wonder if anyone could finally give an hint, maybe marked as spoiler, to what I think is the most elusive easter egg of this mod.