This will be effective as long as it isn't widespread, but there are much simpler CAPTCHAs you can use to avoid spam if you want to take the obscurity approach.

This will be effective as long as it isn't widespread, but there are much simpler CAPTCHAs you can use to avoid spam if you want to take the obscurity approach. For example, I've had great success with simple traps that just make it hard for automated programs to figure out the input form. That approach is completely invisible to human users. But if you do go for obscurity, you're going to lose once your technique becomes widespread, or once your site becomes big enough to be worth targeting.