Re: fyi: WP-reCAPTCHA works fine with BuddyPress
Hey Peter, didn’t see your post until just now.
You’re right that documentation is sparse, but that’s up to users like you and me to add it to the BuddyPress codex.
BuddyPress uses a different method to validate that’s why you can’t just hook in the WPMU validation function that WP-reCAPTCHA uses.
Like I stated above, look for clues in /buddypress/bp-core-signup.php. Check out the global $bp variable, especially $bp->signup->errors. This is what you have to use in place of what the check_recaptcha_wpmu() function uses.
FYI, I’m not using WP-reCAPTCHA.
Might I suggest using a math challenge plugin? It’s more user-friendly than a captcha plugin.
https://wordpress.org/extend/plugins/wpmu-block-spam-by-math/
[EDIT]
Here’s another captcha plugin that supports BP:
https://wordpress.org/extend/plugins/super-capcha/