Hello jbyway
This is more a Microsoft question, but here are my 2cents.
On the hub servers, create a journal rule to journal mails only send to specific users (see
http://www.msexchange.org/tutorials/Exchange-Server-2007-Using-Journaling-Rules.html)
There are some more rules. You might want to consider journaling from the hub servers instead of from the maibox servers.
Perhaps you can create a rule that captures mails FROM certain users, but I am not sure on that. I would first check with MS to see what they can offer.
Regards. Gertjan