You could give each client a set range of source ports so you determine by the log of the source port used to know which client it was. You could rotate the source ports between the clients at a set time and log the new distribution.
The copyright claimer has also to provide the source port used.
This will limted the number of connections at the same time for each client. You then only have to log which client used what port from login time to logoff time or time of redistributing of ports.
Please don’t log more data of your clients for this.
Edit: see starter as http://forum.mikrotik.com/t/how-to-minimize-cgnat-logging/135885/5