The description in the post sounds exactly like how the original WP Hash cash plugin works. That sort of route is extremely effective, but won't stop everything. Combining it with a cookie check and Akismet will block an insanely huge amount of spam though. But even with that combination, some of the smarter bots will still work their way through.

I'm plodding away trying to implement some additional protections on top of all that into a test version of my own plugin. So far it's working quite well, but you need to be darned careful you don't start blocking legitimate users once you push the envelope of spam-protection too far :/