I was testing how Blender 2.5x indirect lightning system works with textured emitters. At first it seemed like it doesn't work at all, especially with images, but apparently you just have to subdivide the mesh enough times to cover those emissive pixels.
So yeah, of course I ended up making a new test/demo of Dynamic Paint. In this one I used two high resolution canvases to cover all the geometry. For rendering I had to subdivide them to total ~1 million vertices to emit light properly.