Just export as a quicktime conversion - animation, with 'millions +' selected.
In some versions there was a problem with exporting alpha once the sequence was rendered, so if the alpha channel gets lost, unrender your timeline and try again. I can't recall which versions of FCP were the problem ones.
Also, I guess there could be an issue with the animation codec depending on the version of Avid you're sending to. Some more Avid intensive guys here might be able to fill in that end of the equation if you know the version number.