there are auth plugins and now captcha plugins. i think activation should work with plugins as well. it would not see as many plugins as captcha but it might see more than auth plugins.

example of mod that should be a activation plugin:

http://www.phpbb.com/community/viewtopic.php?f=69&t=753485

3.0.6 gets captcha plugins. maybe 3.0.7 could get auth plugins? if not then 3.1.x but i think 3.0.7 would be better. i only post here because i think report be closed if posted in 3.0.x tracker.