Make the "Mail To" option in Addl Addr work with mailing downloads

We have many members who prefer for mail to be sent to their PO Box or private mailboxes. This means they are assigned to the wrong political districts, and we then can't target them accurately for political mailings, action alerts, etc. This would not be a problem if we could enter their PO Box/PMD in the "Addl Addr" box and check the "Mail To" box for that. (My understanding is that the Mail To box doesn't work.)