// My bussiness code:
...
Session unitOfWork =  JpaHelper.getEntityManager(em).getUnitOfWork(); 
unitOfWork.setProperty("user", user);	 
em.merge(entity); 
...

// My entity listener code: 
class MyEntityListener extends DescriptorEventAdapter {

 @Override
  public void preUpdateWithChanges(DescriptorEvent event) {
    Session unitOfWork = event.getSession();  
System.out.println("User doing the merge  = " +  unitOfWork.getProperty("user"));
  }