In the thread you link, the responder suggests checking the Privoxy FAQ.
In that FAQ, under Troubleshooting, in the section entitled "5.6. I cannot connect to any FTP sites. Privoxy is blocking me" it states:
Privoxy cannot act as a proxy for FTP traffic, so do not configure your browser to use Privoxy as an FTP proxy. The same is true for any protocol other than HTTP or HTTPS (SSL).
Will Privoxy ever proxy FTP traffic? Unlikely. There just is not much reason, and the work to make this happen is more than it may seem.
So going by that information I'd have to say:
"No, there is (still) no way to configure Privoxy to allow FTP."