I am having a VERY similat problem in that I want to manipulate the header in regards to the Authenticate header variable. You are correct in that the suggestion above will not work for you.
My problem is I am trying to force the killing of an already authenticated session, so I can re-authenticate after time. The problem, is that the server caches the Authentication header and my page simply logs the bugger right in. I am using an external LDAP for auth not .htaccess though.
Same diff.