The odd thing is I can't get it to load with Magic Launcher at all. Even if I just choose test, it'll still crash! I'm positive the only change I've made to minecraft.jar is to have deleted the META-INF folder. I haven't transferred any files from Optifine, HD Skins nor Forge into minecraft.jar.
However, it does work when I try MultiMC as I did what you suggested and zipped the avc.class file and put it into the jar mods. Even though it shows it's loaded along with the other mods in-game. My HD skin doesn't show?
Update: I've got it to work with Magic Launcher. I activated HD Skins as an external rather than internal mod and it loaded it. However, when I join my local server (on the same machine as the gaming one), my character still doesn't have the HD skin heh!