I wonder if some sort of progressive dither or fade is practical to implement and fast enough (and visually useful)? If I wasn't at work right now I would have a look at the code.

I have some experience with Canvas rendering but, like this, just in straightforward layers. I don't think a dither would be practical, a fade might be using a mask and setting the alpha, but you'd still have to detect all the overlap.

I'd say the answer to my question is a 'nope'.