In a discussion with French developers, it was apparent that the proposed colocation choice implied that, so as to minimize bandwidth to the US, the logged-in users should *not* be routed through the European squid proxies, but should be redirected to the US proxies.
The simplest solution for this would be to have two sets of hostnames:
language.wikipedia.org for non logged-in users (where language = en, fr, de, jp, whatever...) language.logged.wikipedia.org for logged-in users
The former would be redirected to the latter when they log-in, and the latter to the former at log-out.
Only the former would be subject to DNS answers depending on the geographical region.
This seems a very easy solution to implement.