I think that the site who command should be handled by the FTP server...I mean sure you could write a script but that kinda defeats the purpose... glftpd, r*, bulletproof...all have site who commands part of the ftp server.. :/
A script does allow customization, but thats what xwho/swho/etc are for and the normal who should be handled by the ftp. This is how I see it, but i guess I'm the only one who thinks this.