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

I've got a mu install on subdirectories:

hubdomain.com/site1 >>> site1.com
hubdomain.com/site2 >>> site2.com

Need to be able to do this

somestring.site1.com >>> should load my custom pluggin page like hubdomain.com/site1/custom/somestring

Problem is, if I try to access somestring.hubdomain.com/site1 (or will be somestring.site1.com once live) it redirects to hubdomain.com

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 hubdomain.com