JsonTokenExtractor is a common Extractor for a lot of OAuth Providers. It's not nice to modify it to support non-OAuth extension of only one Provider.
What's wrong with creating subclass "GoogleApi20JsonTokenExtractor"? Rename later to "OpenIDJsonTokenExtractor" may be.
I'm planning to create maven submodules. Core module and Apis modules. In that case, core module shouldn't know anything about OpenID, until OpenID token in response becomes a part of the OAuth specifications.