A crucial part of making this a successful anti-spam system is that it is a moving target. Every user of the system must be able to write their own questions. If that happens, the spammer's task is intractable. But if there is a central site serving these, it will be worth the spammers' while to just hardcode the patterns and write a little bit of logic to parse and answer them.
Now, there's a fair bit of interesting UI design in the question of "how do I get a non-programmer to write what is in essence a very small program". My proof of concept used some cute Perl-isms to basically construct a mini-language that was restricted enough that an inexperienced programmer could "script kiddie" their way through it, and I think this is the right general direction, but you'd need a fair amount of work to really make it accessible to the masses.
(Other crucial points that he gets right: it must be text based; it must have questions that hinge on natural language understanding but not be otherwise difficult; and it must have questions that are really question templates each of which can generate infinite numbers of question instances.)