Not sure if you want to label it as a “captcha alternative”. In most cases I’m sure the captcha is used because they want a real person looking at the page (and the ads on the page). In this case it seems more like a way to keep either bots or people from doing nothing but consuming content (or hacking) without giving back something of value. Either way I really like the idea.
Other ways, in theory, I think you could do this kind of thing are torrent ratios (e.g. hosting one or moreLinux ISOs), general archiving (e.g. you get asked to return a random range of bytes from a file you’re supposed to be backing up), you run a weather station that reports temperature to the National Weather Service. You might think about a more general framework for just verifying if user X has been contributing something of value.