The JavaScipt API combined with rules would be a great way to do this. You could store the "last" sequence number as a property on the folder itself, then use the JavaScript API to get/set that value on incoming docs via a rule. http://wiki.alfresco.com/wiki/JavaScript_API
Just a reminder about the implications of doing this should you ever enable peer server-to-server replication - you could easily create conflicts as different servers increment to the same, next serial number and then try to sync.