Nullclient with root aliased and user accounts masqueraded...

Sorry for the subject, it's a strange nut I'm trying to deal with here.

Background:
  Two domains are in play, foo.com and bar.com
  Client machines are of the form host.foo.com and host.bar.com in fairly
    equal numbers.
  Unqualified email sent from a any host should have one of two things happen
    depending on the address:
      1. If it's root, use /etc/aliases (or virtual_aliases) to expand
         recipient to root+hostfqdn <at> bar.com and the from should remain as
         root <at> hostfqdn.
      2. All other email for local user accounts should be masqueraded to
         user <at> bar.com.

As an example, take my workstation with a fqdn of coin.foo.com.

If I send unqualified email as root to root, it should send email that looks
like this:

  From: root <at> coin.foo.com
  To: root+coin.foo.com <at> bar.com
  Subject: ....

Email sent from the same host as root to user robert should look thusly:
  From: root <at> coin.foo.com
  To: robert <at> bar.com
  Subject: blah

I've been poking around for several days and tried a number of different
configs up to and including using recipient_cannonical_maps to no avail.

Thanks,
  --Robert