Sorry, taking over the post. Posted it with our company one initially.
Well I'm not sure how you would get the original domain if when you go to the login page on the mapped domain via the htaccess
Example:
Mapped domain: http://mywebsite.com
Original domain: http://123.mynetwork.com
Go to: http://mywebsite.com/wp-login.php but I can't redirect to http://123.mynetwork.com/wp-login.php without knowing the original domain, which I don't believe htaccess can get.