This is probably not particularly helpful, but AFAIK, WCCP doesn't work for FTP with MWG so that's not an option.
Interested in hearing about how others are dealing with this particular situation.
Actually that is a bit helpful since I've been trying to do this via various types of redirects through a firewall and so far have been getting stuck. In one scenario the FTP client sends a SYN packet to the external FTP server (ex: 126.96.36.199:21) the firewall intercepts this and forwards it to the MWG7 FTP proxy service (10.1.2.3:2121), the proxy service sends a SYN ACK back to the FTP client but through a different IP address and port number than what the FTP client sent the SYN packet to, and so naturally the client system sends a RST packet back since the SYN ACK response is not an established connection (its sending a SYN to 188.8.131.52:21 but is getting a SYN ACK from 10.1.2.3:2121 instead of 184.108.40.206:21) and things simply don't progress further from here.
In another scenario a FTP connection manages to get established but gets stuck after the FTP PORT command is sent.
Before spending more time on this -- and possibly finding out that its not possible to achieve, especially given the way that FTP works through 2 ports for file transfers -- I figured I'd ask the forum to see if anybody's pulled this off.
The only possible way is with transparent router or transparent bridge. Neither of which i'm very fond of for a whole variety of reasons.
7.5.1 supports FTP redirect using WCCP.