Hash Cash

It would all depend on the captcha options available to that form extension. If it uses the system captcha (the one configured in global configuration) then it would be possible to implement mine as the default captcha for this form.

From what I can tell, the very little spam I get in my forum is made by people, not bots.....if that's any indication of the effectiveness. I manage the forum by myself, and only once a month.

The hash is calculated on page load. As computers get more powerful, the difficulty can be increased - but be mindful of the lesser computers that might be browsing your site. If you watch your CPU usage, you'll be able to see when the calculation is complete. The default was sane at the time the plugin was created, but computers haven't gained much power since then. It's best to do some testing with moderate devices before setting the difficulty too high.

Also, yes, if it's set as captcha in global config - it will work with any form that implements captcha. It runs on all forms on my site....did you notice?

Well, my captcha does work with the Joomla contact form....