By definition, HTTP Authentication is something that is requested by the WebServer, I doubt you will find a solution that does not result in no IIS Settings being applied.
The web browser will connect to your web site, and unless your server responds with an HTTP response code HTTP/1.1 401 Unauthorized, the browse will not pass through the credentials.
You could try and force a response code of 401 and set the header
WWW-Authenticate: Basic realm="SomethingGoesHere"
Then the browser will prompt the user for username and password, but will be sent over clear-text to the browser (base64 encoded), like this:
Authorization: Basic YnJpYW5iOmJvYmJ5Ym95
Which is translated from Base64 to:
brianb:bobbyboy
I don't know if you'll have access to the Authorization header from your ASP page, or if the Web Server is going to freak out because someone is trying to pass credentials to it when its not expecting it, but could be worth a try...