You will need to create a real user (i.e. FTP user) for each forwarder that needs filtering. This means that you have to delete the mail account of the user before creating the FTP account. There’s been mention of an easy way to convert a mail-only account to an FTP account, but I can’t find the reference.
Then you’ll have to install your own Spam Assassin software and then edit your .procmailrc file to route through your SpamAssassin and then forward the mail.
This is quite a hassle, though. Don’t your client’s destination mailboxes have spam filtering?