What won't work? Just about everything.  That server is so dependent on the
directory, that everything it does that might require a lookup, i.e. folder
reads, message reads, F/B updates (by default every 15 minutes), store
maintenance, mail delivery, anti-virus scanning, backups, etc... Pretty much
everything the server does or might do. It expects to have a GC local to it.
By itself, if this is not a very busy Exchange server, it might do alright
most of the time.  What's the cutoff? Can't tell.  But note that the way
Exchange works (in practice), everything has to get done.  If it is not
finished right this second, then it goes into the queue. You'll run into
performance related issues and possibly some operational issues over time
vs. all at once.  Exchange *can* operate for periods of time with a GC that
is not local. It's designed to do that. It's not designed to do that
permanently.

I sense that you're trying to justify an exception to the decree of no GC.
If you lose that, I highly suggest favoring the server vs. the clients and
have the clients access the server remotely.  If it later becomes a
performance problem, i.e. users are unhappy, then have them put a GC in the
local site along with the Exchange server.  I would not budge on the
Exchange server and the GC/DNS/DC being in the same LAN connected site.
Preferrably on the same subnet.  If necessary, on the same machine depending
on layer 8 issues in that org (I infer that AD is not managed by you? Or is
that just the location of the DC's that is not?)

Hopefully no black-clad individuals jump in through the roof and take your
servers ;)

Al

Almost everything.

Can you not just improve the physical security (lock the DC in a cage
with no keyboard/mouse?) and put a DC/GC onsite, or buys a rack-bount or
other small server and hide it in a wiring closet?

i can see the thinking, but surely there's more ptentially valuable
information in a mailbox than on a DC if you were thinking of data
theft? Since network access to the directory is unavoidable, you only
have to worry about physical access to the server.

It's really going to be a pain. Depending how good the link is you may
have problems getting exchange to start at all, never mind perform well.

Physical access to the exchange server is as much or more of a security risk
then access to the DC.  The exchange server will have databases that store
company data that is worth as much or more then the directory information
stored on the DC.  I would recommend finding a way to secure the servers and
putting both a DC and Exchange there or not putting either there.

Julien,
You have a challenge ahead of you that may be fixed by pre-positioning
Exchange mailbox content on the local network of the remote office.
I'm a storage architect that also handles some exchange design problems for
my company.
Check out a company called Riverbed.
http://www.riverbed.com/technology/app_streamlining/index.php
Their product can provide TCP WAN acceleration which would include the
Exchange traffic. We are looking at deploying these around our region and are
currently evaluating them in our lab. We are seeing up to 80% improvements on
file access and reduced latency so far.
I hope this provides other options and ideas.