I'd love an answer to this question too, although I think I already know what it is. :-(
If I understand all this correctly, you won't be able to do this. The problem is once you log-in via SSL via the main domain, that log-in won't work on unencrypted subdomain admin pages. You'll just be forced to log-in again as your SSL cert is probably only setup for the main domain.
The rightest way I've been able to find for dealing with this issue includes two things.
The first is grabbing yourself a copy of the WordPress HTTPS plugin which does a great job of keeping all this really clean and simple on multisite install.
The second thing is (and don't hate me) to purchase a Wildcard SSL Cert. They're expensive, but will work with all your subdomains, so you can use WP HTTPS to manage how each site is to use the cert.
Also keep in mind that you're going to want to be sure that your domain mapping plug-in is setup to leave admin pages in the subdomain instead of using the option to access them within the mapped domain [example: https://subdomain.domain.com/wp-admin vs https://domain.com/wp-admin
I'd love to be wrong though. :-)
Wildcard Certs are like $350 a year!
Best of luck.