Good thinking @Traktion. This is similar to how WebID works I think, which is how Solid does single login for multiple services (among other things).
So one way of implementing what you describe is to emulate WebID using SAFE. I haven’t looked into the detail, but hinted at it in my DevCon talk.
If you recall when I explained the content of the blog post in Turtle format, I pointed out that it included a reference to further information about the author of the blog post via a URI: https://happybeing/card#me
That is a WebID, and if I understand it correctly one of the things you can put in there is your public key (though I said ‘private key’ by mistake in the recording 
).