(Composure) Is there a way to make a material alter only the alpha channel in a scene?

I’m trying to use composure to composite a scene over a video backdrop. The geometry is hard edged, so alpha goes hard from 0 (opaque) to 1 (transparent). I want to blend in the video background by making some gradient textured planes which fade the alpha channel only from 0 to 1 without seeing those planes in the RGB channel. Any ideas here? I know composure is a bit niche.