I guess mangle might be used to accomplish the scenario, when all data or specific data is marked by action=log.
Probably, some syslog program allows to filter proxt logs per src-address.
Yes. i agree with that. To see log from spcific ip, u must mangle, and add action=log. And then set log in the system - logging - info=web-proxy. Just add the system logging in there. Have a luck