Typically the computer that is accessing the volume must mount it directly, not via an intermediate. There might be a way to set this up by opening up all of the permissions, etc. so that the volume appears to the third computer by merely logging onto the CC.
LocalTalk bridge only puts LocalTalk access onto an Ethernet bus, so it's not applicable here, other than as a connection method. Since you will most likely need to connect to the older Mac directly from your iMac, LocalTalk bridge will be necessary. Have you been to http://www.mac512.com/macwebpages/appletal.htm ?