MU subdir install + domain mapping.....needed: subdomain help

I've got a mu install on subdirectories: >>> >>>

Need to be able to do this >>> should load my custom pluggin page like

Problem is, if I try to access (or will be once live) it redirects to

Obviously this isn't working. I'm trying to hijack stuff before wordpress gets a hold of it by some htaccess stuff but it doesn't help. Still keep ending up at the root