Finally found the answer to my own question. It uses the 'local_part_suffix' generic router option. I guess they are really more "hints" that must be filtered for. They are described somewhat in Wikipedia:
http://en.wikipedia.org/wiki/Email_address#Address_tags
From: Mitton, Daniel (CONTR)
Sent: Tuesday, October 30, 2012 8:41 AM
To: 'exim-users@???'
Subject: Destination folder in email address
I seem to remember an obscure way of adding a destination folder into the local-part of an email address so that mail will be delivered straight to the folder, but I can't put my finger on it again. Does this ring any bells with anyone?? It was something like:
mailbox<somesysmbol>folder@domain
Thanks.
Dan
