Hi,
Well the few setups of ASA + Ironport that I have seen uses an WCCP ACL on the ASA which already can define which traffic is forwarded to Ironport and which simply bypasses it and goes straight out of the ASA.
Those that need to bypass Ironport are specified in the top of the ACL with "deny" statements.
access-list WCCP line 1 deny ip host host
or
access-list WCCP line 1 deny ip host any
I am not sure if this was what you were looking to do.
- Jouni