htaccess and domain mapping

We have got a multisite install with about 1400 sites.We are using the pro version of domain mapping plugin. We have got a sub directory install and have mapped most of the blogs to their individual sites. Now we have a request where we have to further redirect all of the sites to a single domain but we are unable to think of an efficient and feasible way to do the same. We tried to do some redirects in the htaccess file but they dont seem to be working. At this point I cant think of anything that might do the trick.
For example,
The blog is http://www.abc.com/xyz
we have mapped it to xyz.com
now we want to redirect xyz.com to http://www.test.com/xyz
and we want to do the same thing for 1400 sites.
Any help will be highly appreciated.