That is what I implemented for a client recently. They had a legacy system where they could only upload through FTP. The approach was
First upload
contentfile.doc
then
contentfile.doc.xml
and in the xml put the metadata that needed to ba added to the file.
In the target folder, put a rule that triggers when an xml file is added (hence the upload order, the actual content file is there). Process the xml file and add the metadata to contenfile.doc, and finally delete the xml.