Can the compositor be used to batch process images? There are tools to batch process images already of course, (such as ImageMagick), but this isn't as flexible as Blender's compositor since they typically perform linear operations and don't have the control you get with a node-tree.

Is there some way to take point Blender at a directory of images (for example), and batch process them (using an existing compositor setup).
Yes, one not so elegant (no scripting :) but easy way to go about this would be to just use the Movie Clip node and point it to the directory with the images, pass it as input to your node group/compositor setup and render it out as an animation of stills or output each separately using the File Output node.

The only con or drawback to this approach is that the images need to be in a sequence for Blender to iterate through them.. img_001, img_002 ... img_00n or so you can scrub through in the clip editor or timeline but otherwise it should work as desired.