If I remember correctly, the dungog.net content filter contrib does allow reporting by username. But, you have to activate PAM authentication (also included).
And, IIRC, you'll need to specify the proxy server and port 8080 on each client. I don't think I ever got the transparent proxy to work with DansGuardian. (Depending on your network, this may vary in difficulty. It's quite easy with Win2k/XP clients in an Active Directory network using GPOs.)
It has been quite some time since I looked at it though.