Yes, it's an interesting challenge you have set yourself.
But lots of punbb forum owners will no doubt appreciate any work you do to help defeat spammers and further improve this mod.
Questions I have made up are careful not to feature any answers that might contain text from anywhere else on the page. Each question is also phrased uniquely - there are no sets of three similar questions, for example, as there are in the default set. And if a question requires a numeric response, the answer is always at least 4 digits long
Furthermore, I also recommend that anyone who deploys this mod take a few extra measures:
(a) place the human test question in a slightly different location in the HTML code of register.php than the default (ie place the question above or below it's default location in the example register.php supplied in the mod);
(b) rename the two-pairs of publicly visible form response variables in the HTML of the supplied register.php ( ie name="human_test" ) to something unique to your forum;
(c) rename the human test question legend in the HTML code of the supplied register.php from "Human Test" to something unique to your forum too.
By making each deployment of this mod unique, these measures may make it harder for scripted bot attacks against this mod in future. At least in theory.
Brute force detection could be a useful feature to add. It could be a classic 'three-strikes and you are out' type of system, which can sometimes be tricky to build. Or perhaps you could use something similar to the post-flood control that is already in punbb that just sets and enforces a (variable) throttle on the allowed time between form responses. That might also be effective.