The short answer is yes I believe PB can help you here. That said, I'm not real familiar with the Flash side of things so there may be limitations to what you can accomplish. I'm sure Kevin will chime in with a more complete answer.
This should be totally do-able in Flash. Probably the easiest thing to do would be just to write it and test it. You are right, the kernel itself shouldn't be too bad. Not sure about the memory issues though, could be an issue on lower end devices.
Well that's two of you who've said it's possible.
I guess my only question left then is - is it worth it? Are there bottlenecks that FlashPlayer will impose when getting the data into / out of PB that will ultimately prove just as slow as the methods I've tried? Or does PB have some kind of special "super highway" into the Flash renderer?
There is an overhead moving data in and out of Pixel Bender, but if you are using it for a lot of processing or processing a lot of data, the overall speed is a lot faster.
Best way is to try it and see.