I am pretty sure Google has already seen that trick and worked it out
This is a pretty low-life strategy but if you are in need of free AND good content, you could always place an ad on craigslist or a similar site looking for writers and then require a 300-500 word sample on the topic you need it on. I'm sure you would get at least a few submissions. Kind of crappy to do to someone looking for a job but just a thought.