MS Small Business Server and parent proxy

Has anyone managed to configure an MS small-business server running on NT
behind a dialup connection to connect via a parent proxy only (Squid)?

I've been trying to find the option for the parent proxy (customer at
the keyboard, I at the phone), but it does not seem to be possible at all.
Which is hard to believe (or is it?).

Thing is, I normally block direct port 80 accesses from my customers to
force the use of the proxy, and I'd hate to have to change this policy
just because small-business servers can't use a parent proxy.
Is there some NAT mode of the SBS which can be used instead of the local
HTTP-proxy?

-- 
Sincerely,                                                          srb@cuci.nl
           Stephen R. van den Berg (AKA BuGless).
"I have a *cunning* plan!"