Hi Ali,
you are right, that is only good for 1 message. SAP has explicitly coded that more cannot be done. Please have a look at method EXIT_PROCESSING, class CL_SEND_REQUEST_BCS. Here the Exits configured in SO50 get called. And before it is called they create a new trace instance. So if you call CL_TRACE_BCS=>GET_TRACE you would get a new, empty trace which will be discarded after the Exit was processed.
But look at line 170 of EXIT_PROCESSING - there you can see that whatever your method returns gets added to the log.
Regards
Oliver