This article http://www.thesharepointbar.com/?p=340 how to limit a read on a user in BCS, but we can't find the correct syntax for fba-users. (MS Quote: If the login mode for the user is claims-based, then the value passed is the unique value that identifies the user (for example, membership: username, firstname.lastname@example.org)).
So we asked ourselves how to write qualifiedusername behind the scenes with @Username in a create part of BCS (and see what it wrote). But we couldn't find a way for that either. Thanks in advance. Rob