There are two function in the CharacterFrame.lua that have to deal with tabs, i think if we can modify those a bit we could make it work just right, but alas i dont have a warlock to test it with. So maybe i can help you with the code part and you can test it. I dont know.
There are a few things that need to be fixed to work like the other "tabs", i love how ya'll have done them as just small frames with lower draw levels, however, the browse tab on the AH ui isn't set right, nor are any of the tabs on the merchant windows, they appear above the window unlike their brethren tabs which appear and stay below the window to create a unison effect.
I love your work. It's brilliant. One thing though, I think tabs are acting a bit wierd and inconsistent from frame to frame. My suggestion is to use a TEXTURE for the tab like Blizzard does for the normal skin.