using masks as alpha channel for texturing

One of the things I’ve always wanted to do in blender is simple compositing of images and video in 3d space. I currently use afterFX, but their limited idea of 3D, curve and vector editing drive me nuts!
Using simple planes (imported with “import images as planes”) the setup in blender is straightforward.
On still images the use of alpha channels as transparency masks is quite easy. I can use photoshop to create the mask and export the image as a PNG or Tiff withe the transparency mask bundled in.
The issue is creating masks over video footage.
The movie clip editor in blender has great tools for tracking, masking and feathering. The issue is that there is no way to link the mask output into the shader nodes to use over polygon faces, the only use for those mask is in the copositor nodes.
So far the only way I can make things work for me is by rendering the alpha channel as a black and white image sequence using the compositor. Then I import the image sequence to be used as alpha mask for the texture mapping. Any changes or adjustments require rerendering and at the end of the day is a long and convoluted way to do things.
Is there a simpler way to create transparency masks over the video footage that I’m not aware of?