Hi, We have exactly the same question!
Does anyone find a solution?
According to the specification of Open Authorization 2.0 (OAuth2) framework
authorization server MUST support the HTTP Basic authentication,
but no mention of it in the apex listener Developers Guide.
Yes it supports the HTTP authentication which basically it will redirect you to a screen where you have to enter username and password. Once done, it will redirect back to the defined URL. The issue we're try to solve is to do all of this via programming - not manually typing username and password.
I am stuck at the same place where you were. Did you find any solution to that?