Finally getting to turning on squid3 antivirus and smacked right into this same problem.
Running on pfSense 2.2.5-DEVELOPMENT (amd64) built on Sun Nov 01, with squid3 0.4.1.1,
The filename to edit is different, it's now /usr/local/pkg/squid_antivirus.inc
But editing to change [::1] to 127.0.0.1 now works, and even though the C-ICAP access log still shows ::1, it still passes the EICAR test.
Much thanks for the workaround.