I wonder if anyone in House Cannith would dare join the Cults of the Dragon Below.
The thing about the Cults of the Dragon Below is that they can appear anywhere. People can join an existing cult, but it's just as possible for an entirely new cult to spontaneous spring up, driven my visions of Xoriat or Khyber. I could easily see a Cannith heir gripped by wild and dangerous inspiration, whose charisma and wild ideas wins followers in his own house, who, say, seeks to go beyond warforged and truly manufacture life (which is to say, symbionts)... or to master the power of the Mourning... or what have you. Presto, cult of the Dragon Below.
The key with the CotDB is that they aren't a monolithic organization, and they don't recognize common bonds (and in some cases don't even have common bonds to recognize). That Cannith Heir wouldn't necessarily be sympathetic to the nutty orcs in the Shadow Marches who feed strangers to the gibbering mouther. An outsider might recognize common themes in magic, tools, or behavior, but as far as the Cannith heir is concerned, he didn't "join" anything; he founded his own new thing that's going to CHANGE THE WORLD FOREVER! (insert mad laughter)
In any case, I certainly think you can have a CotDB in House Cannith. Who knows? Maybe that's what caused the Mourning in the first place.
As an aside, though, my Mastermind is worse than a simple CotDB leader. I still have an idea where a Child of Winter masterminds an Ice Age on Eberron (good bye Dinosaurs); and if they stop that, our happy Blood of Vol pontiff-in-an-undead-eleven-year-old-body takes over the role.
Worshippers of the Traveler work pretty well too.