I remember contributing the oauth2 authenticator for restsharp and was accepted. not sure if its still there.
from my experience, its better to write your own authenticator, or much easier just add querystring or the oauth 2 authenticator header. The reason I say is that oauth 2 hasn't be finialized and are not consistent with other oauth2 providers.
Some requires to add ?access_token=... , oauth_token=..... or bearer_token=.....
Since it's just matter of adding one querystring parameter, it is better to consult the api documentation and add it manually.
There has already been around 22 oauth2 drafts. even Facebook is on draft 20. so I think its better not to expect the oauth 2 as part of RestSharp anytime soon.
Another problem is for exchaning code for access token and other oauth2 features. Facebook returns a non-json result, Google returns a json result. Since its all messed up it is better to implement what the documentation says instead of trying to generalize oauth2 in RestSharp.