Interestingly, all it takes to enroll is a shell command:

    sudo softwareupdate --set-catalog "https://swscan.apple.com/content/catalogs/others/index-10.9publicseed-10.9-mountainlion-lion-snowleopard-leopard.merged-1.sucatalog.gz"

I previously stumbled upon this URL via a Google Search. (Remember to use your `robots.txt` folks!)

Aside: Seeing as how this circumvents the normal sign-up process, enrolling this way probably does not comply with the program or software license agreement.