Ok, that would make it more difficult. All of the redirects are based within the domain - there isn't a way to forward from one domain to another within the same portal.
I think the best thing to do would be to write a small utility that does the forwarding for you. This could just compare the portal alias, and forward to a specific home page based on a large case statement.
ie
switch (portalSettings.PortalAlias.httpAlias)
case "portal1.com"
//do 301 redirect to portal1.com/portal1homepage
case "portal2.com"
//do 301 rediect to portal2.com/portal2homepage
You can just put this into a ascx file and create a simple module around it.