You can have a single login point which you can redirect to from all sites in the network, and then redirect back to the subsite after a successful login. We did something similar for the WordCamp.org network:
This will probably only work for subdomains, since you can’t really set a cookie for a different domain. If you need to, you’ll need something ajaxy to send authentication tokens between two domains.
However, I think it’s going to be very tricky to actually access wp-admin of a subsite while not being on that subsite, unless you do some crazy rewriting voodoo shmooodo, $_SERVER['HTTP_HOST'] substitution, etc.
If I were you I wouldn’t even bother, especially if you’re doing this for the sake of renaming wp-admin. There are many effective ways to protect your admin panel from bad guys, and renaming wp-admin is not one of them.