S+ on a loss. I didn't know it was possible but I just got it.

Of course its possible! just less likely because when your team plays bad you are more likey to die to the fed enemy. And if you die a lot you wont get an S.

THe only thing you gain on wins is key fragments at a random chance that decreases.

S ranks apply on wins AND losses. Imagine how frustating it is having an S rank, but not earning it because you lost? Basicaly, being the best of the worse also benefits you.

Done it three times with Rek'Sai

Very easy to get an S+ if that's all you are going for. Power farm and don't die seems to be the main pre requisite. Also all that gold from power farming helps you get easy kills. Minions and kills however doesn't winneth the game though its the 50 extra gold for the nexus. :)