I’ve been looking for a software package like this for quite a while. Unfortunately it looks like it hasn’t been updated in awhile, so I’m going to review the code.
Thankfully there is a debian package. I tried it out and it does what I want: it proxies FTP users to different real FTP servers base on the username. :-)
UPDATE: ftp.proxy is working out really well. I’m looking forward to trying it out with a heavy load. Speaking of which, I need to try it out behind a firewall.