You're exactly right that the EntityManager on the server side does not have any knowledge of the properties of the client EntityManager. The client EntityManager is not ever transmitted to the server - during a save only information concerning the added/modified/deleted entities is sent to the BOS.
Some possible options -
- If you can, add these audit properties to your Entities, and maybe use an injected base type, a complex type, or other inheritance to provide a common means of working with these fields.
- Create a custom IPrincipal implementation which includes the audit info. You'd probably also need a custom ILoginCredential to initially pass the fields to the LoginManager, which would return the custom IPrinicipal. This principal would then be available on both client and server.