I think you actually have to get into the HTML on your main page.<br>In the header area of your default page (index.htm or index.html), but a little JavaScript that looks at the actual page requested and then re-routes the browser to that page.<br>In this way, the user is sent to the page requested, no matter what the main page is named.
or
Change your parked domains to addon domains and you'll get exactly what you want. :-)