cike - You could use the WebUI context object which would return the registration page URL webUi.getUserRegistrationPageUrl("/")}
and in place of the "/" you can enter the return URL, which can be set as "postmessageworkflowpage". You will have to get the actions whether a user is coming from clicking the kudo or from reply/ post page etc and set your triggers accordingly. As an FYI - please take extra precautions while working inside the page init script, one wrong move and your stage would be unusable and you will have to get in touch with support to get it back.
