I think that you should be looking at creating a custom Authentication Provider or finding a user management module that authenticates against the individual databases. Authenticating against one of a number of databases might be a stretch, but there are some member management things around.
Mitchell Sellers at Iowa Computer Gurus has a somewhat general purpose module. You have to provide a couple of methods for the code, and you might be able to use that to search one of a number of databases.
If you couple that with something like Dynamic Login from Data Springs, you might be able to "easily" pull profile information and create user accounts. I would also recommend looking at how this is done in the Active Directory Authentication Provider which can create user accounts when an AD user logs in for the first time.
Also, look at some of the modules from DNN Masters.
I think that you may have some custom coding to do but with some thought and consideration, I think that this can be done in the context of a custom Authentication Provider. They are "relatively" easy to create.
And, you can easily include configurations that would disable a member organization so that none of their members could login if their organization ceased to be a member.
If you want to contact me via the Contact Us form at
http://patapscorg.com, I'd be happy to expand on some of these ideas.