I have not checked into how rare item mechanics work in BL2 yet so I apologize if I am wrong about this.
I would try to make the game appear to have four players and have a rare loot drop relic on. The relic can be obtained with the collectors edition pack and this guide gives instructions on how to make the game think there are 4 players.
This just might increase the quality of the drop itself, rather than increase the chances of it dropping... I am not sure. It sounds like its worth a shot to you though. I got my skin from Caustic Caverns, stepping outside the door past the weapon machine before the end of Minecart Mischief outside deep core #6, I was just checking out the area I had been before (I am on steam though).
The person to ask about the rarity/chances would probably be Daemmerung... he (most likely) will know. 09:35, July 27, 2013 (UTC)
Hmm... thanks Ill try and see what happens. Crossing fingers I never have more than two players on at a time myself included and I don't want to open it up for public because I scared they might take it lol. And I just made another character to get another rare loot drop relic... I sold mine in anger because nothing EVER dropped. Anyway Thanks back to work I go... well on Borderlands.