This is one of those things that I never thought of but came up in a discussion with a support subscriber. Can registration be validated based on whether a user is subscribed to the MailChimp list or not? If the user is subscribed, allow registration but if not return an error message.
The answer is yes, that’s quite possible. WP-Members allows custom validation via the wpmem_pre_register_data action hook, and the MailChimp extension has an API function (wpmem_mc_get_list_member()) to check a list subscriber’s status. The combination of those two things can be used to accomplish this in a simple code snippet.