Well, I am undefeated in Cube drafting. I think that Mirror Entity is a windmill slam first pick. I highly recommend odd play mistakes on the part of your opponent. Odd play mistakes are a big part of Cube though, since you are dealing with strange interactions between powerful cards that you've never played together before. I drafted a moderately fast GW beatdown deck splashing red for Fireball. Channel also got in there but never combo'd with Fireball. Why not? Because Mirror Entity is actually a much better combo with Channel.
I started the draft off thinking I would probably force Green a little. There had been a lot of talk about how weak Green was, but I did feel like there were a lot of bodies there. It seemed like people might get a little overly enamored of 7 mana bombs and powerful spells and forget that creatures pretty much rule limited. Especially 4/4s for 4. To give you an idea of how committed I was to this plan, I first picked a Werebear. Not great, not terrible. I managed to end up passing some HUGE green dudes (Kodama North, Verdant Force) because I didn't feel like I had enough accelerators. Instead I got a bunch of efficient midrange stuff (Hierarch, Arrogant Wurm), some pump (Rancor, Armadillo Cloak) and a bunch of White Knights (3 or 4 first striking 2/2s). Match one I pulled out even though I managed to grossly misplay Fireball. RTFC. Match two, I donkeyed away game 2 but the captain (no relation) was horribly manascrewed game 3. Match 3 game 3 was interesting. My opponent was outdrawing me with Bob and had like 7 creatures, only 2 of whom were non black. He had a Kami of Ancient Law stolen from me with Ink-Eyes and a green dude. I had 2 pro-black dudes, but they had to play defense because he has chumps and I die on the crack back. Finally, we reached the point where he could alpha strike for the win next turn. I was on 11, he on 10. I have Mirror Entity, Mystic Enforcer and White Knight. I draw for the turn. It's Rancor! I immediately play Rancor and Channel. I declare my attacks and he immediately cacks my Rancor with Kami of Ancient Law. Oops, blown out by my own card! Except, he no longer had chumps for both pro:black dudes. He was tapped out, so I channel down to 1, dealing exactly enough damage with my 10/10 White Knight. Bad beats, man.
I am not sure how viable it would be to repeat the strategy. Hierarch and Armadillo Cloak provide a pretty strong argument for going GW, but man did I ever want some freaking removal. Lifegain off Essence Warden, Hierarch and the Cloak were all relevant at one point or another.