I wouldn't worry too much about 8-bit render target support, because you can use a 32-bit render target to store 4 gray-scale shadows, one in each channel if you're only going to project them. Of course, this probably also extends to cube maps too - you could use a vertex shader constant to identify the cube map face and then store 4 * 6 = 24 gray-scale shadows in one cube map which should help, at least for batching purposes.

well for now I'm using FLOAT_64, and store in first 2 values my mean and mean difference for self shadowing and in z value a color, which I can use to project onto other objects. Like this I can blur easily, and have soft shadows everywhere.