Allowing the endpoint URI configured in Office to select a particular “tenant’s” active STS, using parameters placed after the formal endpoint name.
e.g. metadata exposes http://foo.com/issuer.svc/office365, but
office SP would use http://foo.com/issuer.svc/office365/DEMO.
The trick is to let attribute the STS callback classes with a service behavior that an capture that endpoint URI actually cited by Office SP – so it can then switch to per-tenant configuration parameters.