I have an instance coming up where I will need to take down my billing software for maintenance. For my Dialup Users I can login and tell the NAS boxes to allow anything to authenticate. Is there a way to tell my Mikrotik to allow any PPPoE users in while I am working on my billing software?

It depends on whether your clients have static IP address configured or not, if not there is nothing you can do.

you need some fake raduis, to accept all requests =)