Publishing Website with pre-assigned credentials?

Is there a way I can publish a website and force the anonymous
connection to use a specific AD account?

We have a web-based calendar program that uses AD authentication to
identify the users, but contrary to their claims it doesn't seem to
support true "mixed-mode" authentication.  

In order to have a "guest" account, we either have to create a separate
login account for each of the hundreds of users in the calendar program,
or utilize an AD account with guest privileges.

So, is there a way (via ISA2004) to force incoming web connections to
use a specific AD account to authenticate to the webserver?

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