Hi Xander,
I'm not quite sure why you want to achive this because you always get the correct alpha when you render the beauty pass. If you want to have alpha seperated into a the colour channels just runt fcheck - open animation - press a to view the alpha channel and then save the animation. Fcheck will save them with the alpha in the colour channel!
Hope this helps!
/Chris
Beauty is worse than wine, it intoxicates both the holder and beholder - Immermann"