Have you tried this before? As far as I can tell, tmux expects to pass the client's terminal fd over the unix socket. When I try it, I get something like this in strace (passing fd 7):

  > sendmsg(5, {..., msg_control=[{cmsg_len=20, cmsg_level=SOL_SOCKET, cmsg_type=SCM_RIGHTS, cmsg_data=[7]}], msg_controllen=24, msg_flags=0}, 0) = 16

And then the client relays an error:

  > open terminal failed: not a terminal