Prior to the new codex, I counted Daemonic Posssession on my dreadnoughts as the Rubric Sign. I assumed that while it'd vary by the individual within the dreadnought, a Thousand Son with psychic powers would probably still be able to use them from within the dreadnought's sarcophagus, and there wasn't really a good way to represent that, so it seemed easier to just make them Rubric dreadnoughts.