Not sure if this is intended behavior or not, but it seems like I can fight alongside the Remnant if they get into a battle and my Transponder is off. In this particular situation, I entered a system with my Transponder off to find a Bounty target pursuing a small, "Dormant" Remnant fleet (and ignoring me), at which point the two fleets engaged. When I entered the battle dialog I expected to be rejected by both sides - but instead I was greeted with the Remnant fleet of all things offering to aid me in battle.
(Just to be sure)