Due to the potential for issues like false positives this would probably be best as a mod -- when something is in core it gives the impression that you can just enable it and not have to worry about it creating problems
It would be a lot easier to maintain the question list if you could look up which question a particular user answered to register their account. That way, if a bot gets in, you can simply delete the compromised question instead of having to 'roll the dice' and replace all of them (most of which are ...
I have about 60 questions in my Q&A config, and while it does its job well, I noticed one or two spammers got through. I need to find out which questions they answered so I can remove the compromised ones. I haven't been able to find this information.