After searching for a day and a half, I finally found the answer (on Metalink, of all places):
Defining Queue Security
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The following setup of defining queue security will allow a user (such as User A) to view another user's messages (such as User B) in a specific queue such as the Personal In Basket Queue in the Work Center.
1. Go to Workflow User Defined Code, select Employee Task Queues (UDC table 02/MB) and verify that the Personal In Basket value has a "N" in the Grid Column Hard Coded. If it does not, change the value to "N". Only queues with a value of "N" in this Hard Coded field will appear in the Queue Security application.
2. Go to Workflow Management Setup, select the Queue Security application (P01135) and click on Add. Enter User A's address book number in the User field and User B's address book number in the Group field; under the grid column Authority Y/N, set the Personal In Basket to Y. This will allow User A to view only messages in the Personal In Basket queue of User B. Follow these same steps for any additional queues where you want User A to be able to view User B's messages.
3. Sign on to PeopleSoft software as User A, go to the Work Center, override the Address Number to User B's address book number and click Find. The Personal In Basket queue should now be available for User A to view.