The reason I posted that link was because I don't think you can do what you are wanting to do with proftpd, not that proftpd isn't a great FTP server. I use it on a few of my servers but I also sometimes need more flexibility which proftp doesn't have. pure-ftp is a secure and stable alternative which does all that you are wanting and more. maybe an alternative, maybe not. If not, sorry I told ya. Good thing about linux is you have many programs and options to choose from, its up to you if you choose not to use them.
However, if you do find some tools that do what you are wanting please by all means post them so I know in the future.