The 'trick' is to dig up the proper XML schema for the Read XML response in Final Cut Server. I don't think the XML schema is an officially published type of thing. You gotta try and leech it out of that FCS Web integration sample Apple put out a few years ago. With the schema in hand, you can generate an XML file for each clip. Then the fun begins ...
You gotta set up a workflow in FCS, so that as each clip is ingested (by a Scan or a Watcher), it pops out an XML file for that clip. The only important data in those popped XML files is the Asset ID and the Title, which will be the file name of the clip. You have to give the original XML files, the ones that you generated from that text file, filenames that match what FCS is going to give your clips and the XML files that it pops out on ingest.
Now, you have two XML files, (in different directories of course), with the same filename-dot-xml. One has the 'live' FCS Asset ID, and the other has the metadata you want to apply to that asset. You have to execute some kind of script that will take the data out of one, and the Asset ID out of the other, and place it into a third XML in the schema format that will allow it to be processed by the FCS Read XML response. Then feed that third XML file into a Read XML Watcher and watch the metadata get updated for that clip.
Use a custom metadata field in conjunction with a Subscription, in order to trigger the XML files on ingest. Just stamp anything in that particular Clip Watcher or Scan to set that custom metadata field to some value that will trigger the Subscription and the Write XML response within it.