There should be render layer operator called .buffer_layer(), and it should work like this:
bpy.context.scene.node_tree.nodes["Render Layers.002"].buffer_layer(1,56)
what it should do in the example above is render the Render Layer.002 at frame 1, and output this image to the compositor node tree, without having to re-render the entire render layer for the following frames until a next frame is listed, in this case layer 56.
What you guys think?