I've been combing through the console (and other) commands trying to find one to set a character as rideable; No luck so far. There are a few papyrus functions that seem hopeful, but I have no way of accessing them yet. (I would like this because it seems like things set as horses by default in the game don't usually show up as other creatures).

Regardless, the TC command changes everything; As long as the Creation Kit or SKSE exposes a command to parent the player to another actor's bone, we'll be able to reinvent riding (and more types of it) if hooking into the current system doesn't pan out.