How to include an opt-out link (with token) in a reminder?
This can't be done - because that token was created with CiviMail in mind.  There was a sense that scheduled reminders wouldn't be used for something that required an opt-out - but of course, people always want to use the software in new and interesting ways!

Unfortunately, I don't think you're going to get this one unless someone writes an extension to do so, or submits a patch to core CiviCRM.