This, in my opinion, is completely unnecessary. Why is it so hard to simply implement SSL (which is a server configuration thing anyway, not related to phpBB itself) and be done with it.
I'm going to downvote this one, it seems like bloat, this would be best as a mod if that's what you want, as personally, I wouldn't use it. If I wanted SSL, I'd configure my server to use SSL and it would use SSL on the entire site. Considering that most people do not even know how to configure an SSL certificate themselves, and that the most that do are probably more capable of setting up a more complex system like you propose, why does this need to be added to the core?