I came across this while having trouble updating artwork before a document save using kAISaveCommandPreNotifierStr.
The notifier does not seem to be sent when the user clicks Save in the "Do you want to Save?" dialog when closing a modified document. That is, it only seems to be sent when the Save is invoked from a menu command.
kAIMetadataSyncNotifier gets sent in both cases though and solved my problem. Thank-you very much A. Patterson.