That sounds like it would work but also be a bit of a pain in the butt for you, Shamus, as well as kinda harsh for people who gotta ask a question RIGHTNOWRIGHTNOWRIGHTNOW.
One thing that has worked reasonably well for Chiba's own forums (which are also phpbb based) was to add a required "security question" to the initial registration form that anyone associated with the JET program should be able to handle without difficulty, but random spammers usually can't be bothered to spend time looking up the answer to.
Ours is "What does the acronym ALT stand for? (Remember to capitalize the beginning of each word.)" and so far, only one person has ever complained of having difficulty signing up (and he later admitted to being a re-re).