The ConnectInterceptor is the best way to do that kind of thing. At one time I had considered adapting ConnectController and ProviderSignInController to allow for arbitrary parameters, but set that aside because I determined that the ConnectInterceptor approach seemed best for those provider-specific parameters.
Now the question is whether or not the GoogleConnectionFactory could automatically add a ConnectInterceptor to do the job so you wouldn't have to do that. I could certainly see Spring Social Google providing a ready-to-configure implementation of such an interceptor so you wouldn't have to write the implementation yourself. Having the connection factory involved in wiring up such a bean, however, isn't so clear to me. That seems a bit out of the connection factory's area of responsibility.
Spring Social Project Lead