ideally this should be flexible enough, with a user-id mapper used when transferring events and requests to the reco engine.

Usecase: a site where users are created on-the-fly from ldap or other existing systems, but buy events are imported from previous site historic data.
In this case the user-id is not known beforehand when importing historic buy-events, and we store in the remote-id field of user object its previous-site-user-id when we create the user.
This way it would all work, if only the extension allowed us to use remote_id of user as the reco-user-id