Uh oh - since I started routing everything here through a FreeBSD router as well (though putting the PPPoE process on the router rather than on the Modem is proving to be a pain with Telstra BigPond's ADSL service, but that is a subject for another entry after I figure it out), things like Facebook will fail when they return an IPv6 address. Whoops.
To the internet!
It turns out someone else has already figured out the solution. Simply specifying the outbound interface to an IPv4 address appears to do the trick, and considering I'm double-NATing at the moment it actually makes it fairly resilient:
Unfortunately though, it doesn't work. I'm not sure why, either. However, inside the comments is also another example that does work, and appears as though recent versions of Squid are tailor-made to solve this problem:
That appears to have done the trick!