SEO continues to both amaze and frustrate me. The more money that gets traded hands based on search, the more brutal and intricate the cheaters are going to become. A lot of the black hat stuff we see now is heavy-handed bullshit, but something like this, if done very carefully, would both be extremely difficult to detect and very lucrative. The canonical thing is smart as hell. You could make that change and, unless site owners specifically went looking for it, nobody would ever see. I could imagine hackers breaking into dozens or hundreds of domains and then auctioning off as very small number of SEO links. This would provide the maximum value for the minimal footprint on the site. Heck, take it up another notch and rent out the space.