Tadd,
Sync-Async bridge fits this requirement.
Target web-service (SOAP over HTTP), is synchronous. When a message is sent to target web-service, it will send back a success message or fault message. Use SOAPUI, to test target web-service.
Lets try this, SOAP <-> PI <-> SOAP. There will be request mapping and response mapping. Please configure this scenario and test it with SOAPUI. Sender agreement will have the WSDL details. Send a Tag ID which is already in use. You can will see response (fault message) in SOAPUI.
File can't be used for sync. So use Async-Sync bridge. Now configure a new scenario, File -> PI -> SOAP -> PI -> File. In response, use response mapping developed above.