I 'm a bit late to the party …
Can you tell me which OAuth2 module you are using? Is it GitHub - bshaffer/oauth2-server-php: A library for implementing an OAuth2 Server in php? If so, the library uses a pretty easy to handle OAuth2 server, that implements a verifyResourceRequest method that checks for valid token requests on a resource. The validation of the sent access token will be executed before executing the resource controller action. If the resource controller is reached, the access_token is valid. If not, there will be an exact api error response with http status codes and a detailed error message in json.
Your use case could use this to redirect to the origin service.