I'll bite.
This setup is the most unfortunate that I have seen in a long time, I'm sorry to say. No offense, but how will you consolidate this once you have several BSP instead of just one ... assuming (like it mostly is) that develpment goes on and on.
The simplest way to console your problem is to ask the basis guys (or security) to create a range of specific UserID's on which all the local participants can agree and work with users from said range only.
Everything that you can technically do to approximate what you call "mapping" consists of considerable monetary effort in both, hard- and soft- and wetware. For example: install some kind of LDAP as an intermediary between systems or any other kind of Identity management. It just defies the purpose, really.
Sorry, I know this isn't what you want to read, but there it is. I'll just leave that here.