Really Simple CAPTCHA can be used with the WP-Members plugin in place of the default reCAPTCHA.
Note: This method of adding Really Simple CAPTCHA is no longer necessary as it is now supported directly within the plugin. Install and activate the Really Simple CAPTCHA plugin, enable it in the WP-Members options, and customize using the settings in the CAPTCHA tab. I am leaving this post active, however, as the code may be useful for developers that want more exact control over the Really Simple CAPTCHA display.