I like this idea a LOT. However, the potential for abuse is, forgive me, too damn high. The extension owner can extort sites to not be attacked or attacked less. (a la AdBlock Plus, ghostery(?), etc).
How about if it merely followed links at a rate not high enough to impact the server itself, but enough to poison the analytics well as much as possible, and rather than opt sites in, it just randomly disrupts the 10,000 largest sites.