Actual visions of Mareth that the Champ would recognize as such would indeed have to be tied in some way to Kas or Taoth. Out of the two, Kas is a much more likely suspect, because she has a motive and an established 'in' past the Champ's mental defenses that she already uses to influence his dreams.Maybe it could be tied to Taoth or Kas in some way??? Like Kas gives you a psychic vision of some weird CoC shit that happened to her years back.
That I would have to veto for Very Specific Reasons which I cannot get into.Another idea that wouldn't require the Champ to actually know much about Mareth would be him getting to bed thinking about how Calla is doing (perhaps prompted by some interactions with Cait), dreaming up a wildly inaccurate and wacky version of her misadventures as a result.