To be sure this is a ftp server for receiving incoming ftp and sftp conections? There is decent server software called Cerberus which supports sftp and it uses events which can trigger $U things like launch Uprocs or tie in to $U triggers. Easy to do a mass import of users too if you have these in a spreadsheet.
Tie this in with the new 'filewatcher' in $U and you should have a pretty good solution.