Thread title. Are we talking Greater Daemon level of power or what?

Furthermore: Could a Lord of Change willingly get itself daemonhosted only to have some crazy gambit involving freeing himself later?

I think that any daemon can become a daemonhost, but the stronger the daemon the faster it will ruin the host.

2nd: I could see this happening if the Lord of Change had a plan to stay in realspace somehow.

1: I know of no limit to the power of a bound daemon. However, the bindings would no doubt need to be stronger, the wear on the host all the more obvious, the summoning ritual all the more dangerous. All that said, I'm sure there's a unpleasantly large amount of radical inquisitors with questionable survival instincts salivating at the mere thought of having a greater daemon bound into their service.

2: Certainly. As even BC makes clear, Tzentch himself schemes for the sake of scheming, and his greater Daemons are no different.

A Keeper of Secrets could certainly hatch a number of plots and intrigues based on him being summoned, possibly even going so far as to whisper into the ear of a cultist to perform some action that occured within eyesight of a young child, subtly but irrevocably altering it's world view. This child would then grow up into an inquisitor, where another plot by the Keeper would see the young child stumble upon a old man who knew a woman who knew a location that housed a fragment of a book that housed the mention of a ritual on how to summon it. Said trail of clues leading said Inquisitor unto the path of Radicalism, urged on by a million tiny things put forth to tempt him onto the path, only to culminate in the summoning and binding of the Keeper.

And then, centuries later, perhaps, another man would stumble upon it, and the Keeper would whisper a truth he had not dared contemplate yet could not deny, and as he recoils in surprise, he bumps a book that falls and cracks open and the wind from it's fall brushes a circle on the floor that disturbs the binding and the Keeper is freed of all it's restraints, yet maintained on the mortal plane. Just in time to go watch the first and only ever performance of the Opera play known as Springtime for Tzeentch.

Just. As. Planned.

So yeah, go for it!