I believe I have seen it, as I've also seen other flagships in other TFO's.
"You don't want to patrol!? You don't want to escort!? You don't want to defend the Federation's Starbases!? Then why are you flying my Starships!? If you were a Klingon you'd be killed on the spot, but lucky for you.....you WERE in Starfleet. Let's see how New Zealand Penal Colony suits you." Adm A. Necheyev.
I have seen the Lleiset join a Romulan Imperial Minefield TFO as an ally to the team only once before. I'm not certain what the conditions are for it to appear.
Comments
Yes, I remember that happening often in one of the Romulan patrols.
Randomly appearing in TFOs though? Maybe I just never noticed?