How do I export from After Effects to FLV with transparent background?

I'm trying to export a simple animation from AE for use in Flash -- export as an FLV. This animation is 20px x 20px -- a small button element.

I want the animation to reside on a transparent background -- so that I can simply import that FLV into Flash and lay over my other elements.

This seems simple enough -- and I know a little about AE -- but I can't get it to work.

Do I simply use a Mask around my shape(s)? (Layer->Mask->New Mask)?

How do I export? If I try to Make Movie, I get an obscure error saying output and transcoder are mismatched. (I set the FLV output settings to RGB+Alpha for what it's worth)

If I try to Export To FLV -- I get an FLV that seems empty and crashes Flash when importing. (I also set FLV to RGB+Alpha here)

This is all on a Mac, under OS/x for what it's worth. AE and Flash CS3.

I went through a tutorial with greenscreen and chroma key -- but this seems as if it should be even more simple.

Thanks for any help!