Hi Jeremy -
The IUser interface is not meant for extension in Mosaic.
What is the purpose of the extension? Maybe we can suggest some other ways to achieve your goal.
Note that you can assign roles and permissions to users on the server:
These roles and permissions can be used to deliver certain tiles to certain users, but the permissions are not exposed to the client API.
I don't think extending the IUser interface would get you much, the Mosaic framework wouldn't populate the extended object.
If you are using the user object for SSO, then you can get the SAML assertion from it using the SAMLAssertion method. It returns a string that is XML data that tells the user, how they were authenticated, who issued the authentication, etc.
I'm not sure about your case, but sometimes there can be additional information as well that come from the LDAP. These are stored in the attributes section and can include things like departments, roles, etc.