This is an interesting thread and an issue I am also encountering presently. There is an authentication service called OAuthAuthentication available (7.2.2 instance), but there is nothing in it that appears to use a listed OAuth Provider as one might expect - and as it likely should. Do we know if this is on the roadmap, or if there is an implementation pattern available?
The option to use REST is also there, but it that feels like it's working around an issue rather than solving it.
Did you get any further with the OAuthAuthentication option? I'm looking at the activity pzOAuth2AuthenticationActivity associated with the authentication service but can't see a call out to the external system either.
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