This stopped being speculation a few days ago.
Over 10 people have confirmed with proof that; the portal spawns, you click it, a few seconds later the scenario queue pops, you enter, the boss is laying there dead and there's an egg you click to get the mount.
I think this could even be done via questbot and doesn't need Rarekiller.
We also would only at least need a way to detect the portal and click it.
There is also confirmation that someone was in a group and found the portal, they clicked on it, but were unable to enter the scenario due to being in a group. Then a minute later they received the mount in the mail via the same way you receive loot 'lost in the void' from a dungeon.
It appears to have the same spawn behavior as Poundfist (not sharing it, but rather the same logic)