You are correct that there is no way you could have a hybrid tachyon/human creature; in fact, if you want to keep relativity more-or-less intact, I don't think tachyons and normal matter could really interact significantly (ie. transmit information between them) at all (let alone across a brane! Remember, we have them in different universes as per your current design). I was thinking that the Tachyon-entities' control of their avatars would almost have to be metaphysical in order to avoid breaking relativity...
What the hell!?! This is total gibberish, if they ever come up with a new star trek series you should write for it ;) What exactly might the Green's Function be for inter-brane or inter-universal tachyon propagation?