Only a vague idea for a workflow that might be supported by a plugin... which may or may not be feasible so don't get your hopes up. Also, I'm not even that much interested in it myself, so I'm probably not going to create a blender script for this myself.
Instead of drawing 4(or more) direction light maps, maybe it's possible to draw a greyscale heighmap version of the image. Maybe it's easier or even faster. I'm no pixel artist and it sounds pretty hard, but so does drawing 4 direction light maps.
Now in Blender: Create a new plane and scale it to the proportions of the image. Subdivide it into the x and y resolution. Apply a displacement modifier, using the drawn heighmap. For the last step, bake the maps you want to another plane with the same proportions and size. Tadaa! Normalmap.
Just a thought for now, so there might be a number of problems in it.