It seems like I was wrong about LimeWire and auto-find sources because LimeWire already seems to do this --- "Does retries and resumes automatically" according to
the ManagedDownloader Javadoc.

Also, a line from the "MessagesBundle.properties" reads: "DOWNLOAD_STATUS_FAILED=Awaiting Sources" which seems to mean that when the downloader goes into a GAVE_UP state, it displays "Awaiting Sources" next to the failed download item.