Maybe what Curethan was suggesting, but I assumed that the Psuke Cants of Calling work the same way as the other schools. One must know the person and the place. I had always thought that the 'place' clause was very specific. He'd have to get the correct bedchamber. Also, the cants of compulsion work on those that cannot perceive the Onta, so I would also posite that this is not a limiting factor to sending dreams. Moenghus probably tried to contact every single Dunyain he could, but because of his 30 years absent, he only knew a few that remained alive, and could guess correctly only a handful where they slept.
The above quote from Bakker kind of points to the fact that Sight was tied to other useful traits in the Dunyain breeding program, so I believe nearly all the Dunyain can see the Onta
(side note, I want a better way to describe see the Onta. Maybe an acronmy... SotO (Sight of the Onta), PotO (Perception of the Onta)...)