Always log proxy usernames

[Tim Bates <tin@xxxxxxxxxxxxxxx>]

We use passthough authentication, but have no access to the upstream 
logs. I vaguely remember seeing some docs on how to trick squid into 
logging the username, but still passing it on. But I can't find anything 
at all anymore...
Is it possible to make Squid *always* log usernames (when present) in 
the logs, but still pass the account details on to the upstream proxy?

TB