Creating a User to access multiple databases
I'm not certain about the service broker que but it is possible to setup a user without a login.  This user will not be able to authenticate but you can still assign the user rights.  The examples below are from the microsoft article for createing users http://msdn.microsoft.com/en-us/library/ms173463.aspx

D. Creating and using a user without a login

The following example creates a database user CustomApp that does not map to a SQL Server login. The example then grants a user adventure-works\tengiz0 permission to impersonate the CustomApp user.

USE AdventureWorks2012 ;
CREATE USER CustomApp WITHOUT LOGIN ;
GRANT IMPERSONATE ON USER::CustomApp TO [adventure-works\tengiz0] ;
GO 


To use the CustomApp credentials, the user adventure-works\tengiz0 executes the following statement.

EXECUTE AS USER = 'CustomApp' ;
GO


To revert back to the adventure-works\tengiz0 credentials, the user executes the following statement.

REVERT ;
GO