Import video (w/ alpha channel) and export image sequence (retaining alpha channel)

Hi. I am new to Blender.

I have a .mov video file that contains an alpha channel. I want to import this video into the Blender video editor, and render it out to a PNG image sequence that retains the alpha channel.

I think I found the correct output options already:

http://i.imgur.com/osNKdHH.png

However, when I render the output, the PNG images have solid black backgrounds where alpha/invisible pixels should be.

It seems like Blender does not realize that the source .mov file has an alpha channel, because if I layer several such videos into the sequencer (all playing together), I can hear the audio from them all, but only the video in the top layer is visible (with the same black background).

Am I missing something here? Is this not something Blender supports?