Hi,
I have not completely understood your scenario here, users dont need to contact the JBE to request a build. JBE is just like any other client to RTC Server and when starting JBE one can use userid/pswd or SSL or Kerberos based authentication. The builds run in the context of the JBE user even though the build requester could be anyone.  A build request is done via a RTC client(like web, Eclipse, VS etc) to the RTC server. If you intention is to control who can request a build, this can be achieved by permissions based on a users role.