Is there any way to list all simple addresses that are in use on a site?
Yes, In the report center there is a report that will show a list of simple addresses used. Check page 135 in the Editors manual.
edited: I checked a cms 5 site and its in there as well.
You can use the following SQL query (EPiServer 5):
SELECT DISTINCT fkPageID as 'PageId', ExternalURL as 'SimpleURL'FROM tblWorkPageWHERE ExternalURL IS NOT NULL