[squid-users] accelerator mode in case where content depends on client IP address

I am looking at deploying squid as an HTTP accelerator in
front of a site which uses an IP-to-country lookup to vary
certain elements of the site's content. Obviously I can't
specify this information in the Vary: header; I could have
the site emit `Vary: *' but that would guarantee that no
useful cacheing is done.

Is there a suggested solution to this? One which occurs to
me is to modify squid to do the IP-to-country lookup
itself, and to add an additional header to the client's
request containing the results of the lookup; the
application behind the accelerator could then name this
header in the Vary: header it returns. I think that would
work, but it's a bit ugly; is there a better (or, better
still, standard) approach?

-- 
``Every revolution evaporates and leaves behind
  only the slime of a new bureaucracy.'' (Kafka)