Just make a Federation-Romulan fleet costume and a KDF-Romulan fleet costume. There is one of each fleet, you can only join once you choose a side at level 10. Once you've done so, your alliance costumes unlock.
The only thing that should be restricted would be any gear not immediately unlocked after reaching lv 10 and choosing a side. I, however, would see no issue in posting a picture of my Romulan's costume with the 22nd century helmet since it is pretty much an optional piece for my crew (since the non-Romulans can't wear it). If there were a costume contest, I would consider it a "Neck Down" contest, and the helmet would simply be there to add flavour, just like scars and tattoos. :3
