The mimic dart (apparently) changes the colour of your nameplate to match the affiliation of your target (i.e. to an enemy, your nameplate would appear green).
I figured the reverse mimic dart would do the same/ opposite (change your target's nameplate to match your affiliation), so to your enemy's team mate their gamertag would appear white rather than green.
I think jumping on a galleon and hitting several members of the crew with reverse mimic darts so there's two to five of you running round the ship would definitely sow a little confusion.
Yes you'd be able to figure it out once you've hit someone, or by reading their gamertag, but as @MisereNZ says, that would still take a second or two, and when there's an enemy on board your ship, I imagine those seconds could add up and become quite frustrating.
I also think it's important not to write ideas off just because they wouldn't necessarily win fights against competent crews. The important question is whether people could have fun with it. I think they could.