Maybe a better solution to the Alexa problem would be to simply create a composite score that combines rankings from Alexa, Ranking.com, and Compete.com (and others?).
I don't know if you can easily suck data out of these other services like you can with Alexa (nor do I know if they even have useful data). In fact, ranking.com uses a CAPTCHA, probably to prevent this sort of thing.
But yeah, go do that.