dispatching on host header - ISAPI or .NET filter

Hi

here is the situation
we have IIS site with multiple host headers

based on host header we need to send to different URL's within the server

my current solution (hate to call it naive :)

I have a small dsipatch web app that takes incoming request takes
SERVER_NAME header
and from db retrieve the URL and do the REDIRECT
It works but some SEO guy says - no redirects!
if a visitor to any of those sites types www.somedomain.com
it should stay in the browser that way when site displays

I tried doing server.Transfer but it cannot be done between diff web apps

Questions:

Can I in the filter or ISAPI or whatever do the work my dispatch does
to send directly to that URL determined by SERVER_NAME

will there be redirect needed

what to use or rather can it be done (preferrably) in .NET filter

Sincerely
GSL