You know what? This thread inspired me to do my own Nuzlocke run on Pokémon Yellow. My Pikachu died shortly after beating Koga, but there really was no problem going on from there.
How far did I get? Lance beat me with his Dragonite which killed my ~lv50 Gyarados, Dugtrio and Kabutops. And my lv 46 Venosaur died against the first elite four guy.
Moral: DON'T EXPECT TO BEAT THE ELITE FOUR WITH LV50s! I learned that the hard way :(
...just thought I would give you a heads up :P