I was looking for a SAML Authority that can consume SAML Attribute query and returns SAML Attribute assertion without login or SSO flow. It is good for retaining user data to make authorization decision. SAML is a XML-based open standard data format for exchanging authentication and authorization data between parties, in particular, between an identity provider and a service provider. The service provider may request for user attributes by sending an attribute query to the identity provider
Looking at the use case I can say i've never seen one that supports this, either way it would be a very costly transaction.
It sounds like you are trying to fetch additional information about the user while they are interactivley interfacing with a downstream application? If that is the case i think leveraging the CORS capability of the users api would be the best thing to do.