Sounds like captchas is something you want to make plug and play, and use some external project that is evolving quickly to stay in the winning side of a arms race. Also sounds like captchas is something you want to be handled by locals, to avoid the situation a chinese wiki with a english captcha.
Is pretty much proved that "small self-made captchas" don't do for something like mediawiki, because attackers target it and is a huge delicious target.
Has experience of people with AI and computer power raise, perhaps this will become a lost battle*. The other option is anon can't edit articles, ...anon edits are invisible and waiting for moderation, ..anon changes are satinified in some way (perhaps not allowing new external links / modiying links ).
* I can imagine the ability of bots to understand catpchas will grown, but not the ability of humans.