C# - URL Routing Across Multiple Subdomains?

I find myself in a difficult situation. We're working on an ASP.NET MVC 2 application which is comprised of multiple sections. It is a design goal to have these sections span across several subdomains. Each subdomain will have its own controller.

The challenge is that our hosting provider's control panel allows two forms of redirection for subdomains, and neither of them seem to fit the bill. The choices are:

Redirecting to URL. Choice is given whether to redirect to an exact destination or a destination relative to the request URL. Redirecting to a specific folder in my hosting space.

I'll try to illustrate the intended behaviour. Assuming the default route is {controller}/{action}/{id}, I'd like the URL http://subdomain.welcome.com/a/b be handled by the MVC Application like http://welcome.com/subdomain/a/b.

The URL redirection could solve this problem, except for the fact that the user sees a URL change occur in the browser. We don't want the client to see the redirection occur.

Redirecting to our MVC apps root folder doesn't work at all. The app doesn't pick up the request and a 4xx error gets passed back by IIS.

edit:

In the interest of finding an answer, I'll simplify this a bit. The "redirect to URL" doesn't do what I want so that leaves redirecting to a folder.

If I'm redirecting a subdomain to the root folder of my MVC App and IIS wont pick up the requests, is this a limitation of IIS or my provider?