Messing around with Sprite Lamp. It is an application that creates normal maps of an image from drawings of where highlights would be if the light was shining from different sides. The first image is my base image. Then the highlights from the left, then highlights from the top (with Spritelamp you need at least two, but you can get better results by also drawing lighting from the bottom, right and center also). Next is the normal map that Sprite Lamp outputted.
Finally, what it looks like when the shader is applied. It actually looks kind of junky, but that's because my choice of base tile wasn't made with dynamic lighting in mind. Overall Sprite Lamp works pretty well. A little more labor intensive than I'd hoped. I think the tool I'd really like to use is Sprite DLight.