Hi @100005240821699,
While I don't know the exact answer to your question, I did move it to the Developers Group area, where someone might have more info. In addition, I do have a couple comments...
I'm sure the technical bits are in place to use Canvas as a login, as that's what the Community site here does... Whether Instructure will allow 3rd party vendors to do this, especially on a global basis like the community does, is questionable though. As an admin for a higher-ed institution, I can say that we wouldn't want 3rd party vendors to be able to authenticate users through our own Canvas instance, as that somewhat applies an official relationship between the institution and vendor. The free-for-teacher Canvas instance is unique in that anyone can sign up, but as you noted, general users cannot have admin privileges, and there is limited support for things like external apps. My guess is that this would not be available at all for the free version, but it may not even be available for a paid Canvas instance. Hopefully someone else will chime in as well!
-Chris