Domain Mapping: admin backend redirects

Is it all possible to redirect all backend administration to the non-domain mapped original URL?

E.G., I have my MU install at http://muinstall.com ....

I create a site for someone, and it initially is at: http://site.muinstall.com.

We then map that site to http://site.com.

I would like users wanting to edit their content to be redirected to http://site.muinstall.com/wp-admin/ .... not http://site.com/wp-admin/

My reason: SSL certificates. I can buy one *.muinstall.com SSL cert, versus an individual one for each new site.

Donncha's plugin claims to do this, but that plugin also breaks the wordpress functions dealing with checking if a user is logged in, so it messes a lot of other stuff I do