this thread has to be one of the most extreme instances of algae I've ever seen. Yuck.

The strong hydrogen peroxide sounds like an excellent idea, though I'm not sure where you would buy it.

I've used DOW Scrubbing Bubbles and radiator flush on a number of occasions. The Scrubbing Bubbles are good about getting stuff out of the rad. I just squirt the rad full, let sit for 10-15 min, flush, and rinse. I would think if you do this a few times coupled with some radiator flush, it should do the trick.

Martin's idea of doing a force flush afterwards would be a good idea. Ive done that many times also with rads while hooked up to the kitchen faucet. I do follow with a distilled rinse to get the tap water out.

Even with all of this, you may want to run an inline filter for a bit once you put it all together and crank it up.