Hasta La Motion!

I’m often find myself in a situation where I want to use mattes from objects in a 3d setup. For example, I have several images with alpha channels going into a scene node and going out through a scanline renderer. After the scanline renderer all alpha channels from the objects have been merged into one single alpha channel. But what if I want to use only one of the elements as a matte later down the tree? One solution is to copy a matte into a new channel before going into the scene node, but then the matte will be blocked out if another object is in front of it in 3d space. Another solution is to use separate scanline renderers for every matte, but that isn’t very efficient of course. Anyone havea nice solution for this?