I'd really like to have an B2Evo expert to answer on this so that I can take a look into making such a plugin, if possible.

There's an Akismet plugin already. This currently only returns karma for the spaminess.
Instead it could be an option to trigger a Captcha plugin instead (there's already a Captcha Image plugin and I've started pulling the meat out there and provide a captcha_base class instead, so that you could easily create new captcha plugins).

Take a look at the Akismet plugin - http://manual.b2evolution.net/Plugins/akismet_plugin. If you want to contribute to it, send me your SourceForge.net account (or some patches).. :)

I knew those two existed and I was looking into it to combine the two. As soon as I have some time I'll dig into it.

I've created a plugin yesterday, which uses a captcha plugin: "Login protect plugin".

You should take a look at this:
http://evocms-plugins.svn.sourceforge.net/viewvc/evocms-plugins/login_protect_plugin/trunk/

This is just a proof-of-concept plugin, but it should help you with the captcha-integration into Akismet.