We could for instance say that a captcha should be filled out every time a worker produces a week's worth of hashes on a standard AMD GPU. The captcha should be part of the hash challenge mechanism so that pool operators would be excluded from being able to fill in one global captcha only when they find a block. This means that exclusive p2p mining should be inherent to the protocol.
The first thing that comes to mind is that people with big mining operations would just hire cheap labor to fill in the captchas. This, however, would make mining more expensive for big operators. For small operators it would just represent a minor nuisance.
Could this work?
Why don't you buy more GPU's?
You just feel slighted that you can't find a block with your wiener rig