asterisk users - Mar 2005 - Local/Remote * Servers, IAX/SIP mix and voice-mail notifications

We currently have an Asterisk server set-up, serving a handful of
sip-phones and sipuras, and connecting to the outside world via one FXO
and various SIP and IAX providers.  In order to conserve bandwidth and
have a fall-back in case our internet connection becomes unavailable,
we're looking at putting * on a hosted server and funnel the calls to
our office, such that:


 DID  DID         DID
  |    |           |
(SIP)(IAX)       (ZAP)  ---(SIP)---PHONE
  |    |           |   /
  REMOTE         LOCAL---(SIP)---PHONE
  SERVER--(IAX)--SERVER

While this (to me) isn't a trivial setup, it should allow us to keep
local calls local (the LOCAL server is behind NAT), have voicemail on
the SIP/IAX DIDs, even when our office is unreachable, and if we deploy
G729 licenses on both servers, we can keep cat the required bandwidth
down to a bare minimum.  The ZAP channel is used for 911 only, no other
calls are placed on it.

What I can't seem to figure out is how to pass the MWI from the REMOTE
to the LOCAL server, so that they light up the proper SIP phone.  Am I
missing something tremendously simple or obvious, have I searched for
the wrong terms, or is this just not possible?

An alternative would be to eliminate the local server entirely and have
the SIP devices register with the remote server directly.  This, on the
other hand brings all the joys of SIP through NAT firewalls with it, and
while I don't expect it to be a problem, I'd lose the advantage of
keeping the remote server out of the media path for local calls.

Suggestions?