Central User base, multiple sites

I'm looking for a way to do the following:
* Have a central site the syncs up user accounts with XML feeds (already have working)
* Have sub sites (aka courses) use the user accounts from the central site
* Need to be able to do this without storing accounts locally in any of the course

Has anyone done anything like this or know of any way of accomplishing this? We have webaccess / cosign going so we have a single sign on system, i'd just like to keep everything central because we need to account for people adding / dropping courses via the XML string that gets updated twice a day.