I don't know if there is anything already available to do this , but I have been unable to see anything on the website or forum.
It would be nice is User Manager was available as a standalone central server. And it could communicate with other Microtik routers.
Essentially have one central location for User Manager, running the "user" info, the webpage, and processing the payments. This unit could then be in a more secure location, rather than out in the boonies, or at a tower, where it can be easily be taken out with a lightning hit.
The central server would then communicate with all the other router units you have setup as a "System" and get the user data, process it in the central location, and control the remote router user there.
Thanks,
Wayne