The oauth client packages (i.e. Twitter, Facebook) all have a requestCredential method on the client, and retrieveCredential method on the server.  They can be (and are) use to retrieve credentials from the respective service, and then those credentials can be stored in other collections.  We have the notion of teams in our app, and credentials for twitter accounts can be stored against the team instead of individual users.

I'd very much like it documented how I can use oauth-encryption for these credentials that don't use the accounts packages.  

I guess I'd also consider it pretty important that existing apps have an easy way of encrypting all existing users...  Either the framework doing it, or with a high quality code sample showing how.  As @awwx implied, it's not completely trivial, so it would be nice to prevent people from making mistakes with it.