That's a pretty hard thing to do. Some web sites will use HTTPS for specific things and not others, so you might end up with things breaking. I don't think that it's really a generally feasible thing to do.
e.g. You've got some site that uses HTTPS for logins and whenever you're logged in, but nothing else. So it's available. Your extension sees that it's available. You may end up constantly hitting a login page for every page even with different URLs because the site sees that you're trying to use HTTPS, but you're not logged in, and so returns a login screen for every URL.
There are more problems with it as well. Basically, I think it's up to the user to check if they're on HTTPS when they want to be. Otherwise, you need to trust the site.
Think about it this way, if you're creating a site that uses HTTPS, you've probably got a good reason and will dictate where it is used. e.g. You can't see certain pages or certain content without HTTPS. etc.