> need to be some degree of agreement and coordination between token producers about what a particular scope string means & agreement between different services (...)

I agree with you on this point. Actually there are already some "common" scopes, for instance OpenID Connect defines some scopes ("openid", "profile", "email", ...): https://openid.net/specs/openid-connect-core-1_0.html#ScopeC...

However I don't think (to be confirmed) there is a IANA registry for scopes (source: https://www.iana.org/protocols)

But there is one for claims in JWT:

https://www.iana.org/assignments/jwt/jwt.xhtml#claims