Transparent redirection CAN process user info.
When you open a browser, and WCCP redirects you to the WSA, the WSA inserts a credential challenge.
When you set up the WSA for transparent authentication, you join it to the Windows domain, and create a NetBIOS name and add that to DNS. The challenge appears to come from an internal server, so IE (and now Chrome and Firefox) will use your Windows auth info to answer the challenge and you get access to the internet...
On a Mac, since its only sort of joined to the domain, and Safari doesn't grab the windows authentication, you get prompted.
I haven't tested this with a Mac in mind, but you may try deploying a CDA. The CDA grabs authed users from the domain, and passes their IP and auth info to the WSA (and ASA if you want). We use it because some apps (Outlook, Bentley licensing for engineering apps, others...) don't handle the authentication when they access the internet (eg "show me the pictures" in an email).
