My understanding is that client_id is for identifying the app in use (in this case rclone).
OAuth always requires a user as well - at least the way rclone uses it with grant_type authorization_code.
This could probably be made to work by using a grant_type of client_credentials but it would require a bit of work in rclone.
BTW You'll need to put the client_id and client_secret in the config file too as rclone will need them to re-validate the token when it expires I think.
If you can get a token with your curl command, you can try putting that in the rclone config file - does that work?