In an effort to help future people that might find this thread, I'll post this.
The only way I've found to scan emails that are supposed to be forwarded to another address is to ALSO create a POP3 account for that forwarder. You could set the space limit of the account to somewhere around 10-50MB each and then make a script that will automatically delete the email from your /new/ folder every few minutes. You'd also need to setup a filter to:
===
Spam Status
begins with
Yes
Deliver to folder
/dev/null
===
to discard the email found to be spam.
I've tested this with cPanel 11 and this works. I'm sure there is probably some way to rearrange your SA config rules to make it scan emails before forwarding them off the server (without requiring you to create POP3s each time) but I don't know what that would be. If someone knows, it'd be a great resource for people.