I believe the ASP Server.Transfer method (as opposed to response.redirect)would be able to do this. If not, you could use the XML DOM, ASPTear, or something of the like to scrape the data from the page instead of actually directing the user to the page itself.
Alternatively, You could created a framed page with a frame at the top, and load the other site in the 'main' content frame. Since the framed page will be hosted at www1.domain.com and the content loaded in the 'main' frame, the address bar will never change.
Sincerely,
Mark