My Starfleet-allied Romulan character is unable to claim any of the T5 Mirror Universe Starfleet ships that come from lockboxes. Attempting to open the ship box fails with: "You do not meet the requirements for this item."
Mirror Universe ships were not listed among the exclusions here:https://www.arcgames.com/en/games/star-trek-online/news/detail/10898183-romulans:-fly-the-ships-of-your-allies
So I'm assuming this is a bug.
I have not tried this with KDF-allied Romulan character/Mirror Klingon ships.