I'm a little confused. Most *nix filesystems save the user as the UID number, which is then translated back to a username by the system. (Via PAM or straight from /etc/passwd in the most simple case). In order for the filesystem to be able to reflect the username used to log into the ftp server, that user would have to be available system wide, not just to the ftp server.
What I'm trying to say, is that I don't see how you can have purely virtual ftp users create files on the system with their own UID. I would look closely at the proftpd system to see how its working. For example, on a GNU linux system, running
ls -n will print out the directory listing showing the numeric UID, instead of the username. Then take a look in /etc/passwd and make sure there is nothing listed for that UID.