I created a plugin to reduce the amount of spam accounts created on a mediagoblin instance by adding a Google reCAPTCHA to the registration form:https://github.com/gsarkis/mediagoblin-recaptcha2
This plugin is simpler than the one in https://issues.mediagoblin.org/ticket/1086
because it only replaces the registration form, not the authentication system. I only tested it with basic_auth. reCAPTCHA display and processing is handled by wtforms-recaptcha.
There's a quirk though: the plugin must be enabled before basic_auth in mediagoblin_local.ini.
Eventually, I'd like to add reCAPTCHA 2.0 support.