How to render only the Alpha channel of an animation

Hi!

First of all I am sorry if this problem has already been solved elsewhere and I just didn’t find it…

I have made an animation of a logo simply rotating and a friend wants me to render the exact same animation again but only the alpha channel of the scene with some additional highlights (optional) and a transparent background. The animation has been done as PNG files and the new files would be used as some sort of mask for an sound input responsive flash file.

I would be extremely happy about any kind of advice.

Thank you!