MAPPED SUBDOMAIN MULTISITE INSTALLATION: SSL enforced only on single, entire (primary) domain

What rewrite code do I need to put in my htaccess to enforce this rule only on the primary domain and have no effect on mapped sub domains?

Since the domain is the primary domain on the installation, it is not possible to simply change the http to https in the site settings, as it cannot be edited.

I have seen a few posts here that didn't address the issue, so I hope I'm not overlooking a solution. I don't want to use a plugin, or hack my database if I don't have to.