google-authenticator for two-factor-authentication support with openvpn client on linux

I am trying to get openvpn client to work with google-authenticator and two-factor-authentication. I cannot alter the server in any way (i.e. I cannot do password/token concactenation via pam mods, etc).

I assume this would require recompiling the source code to include two-factor-authentication functionality from google code, so as to get prompted for two-factor-authentication code, after entering password.

Has anyone done this, or have any tips on doing this?

This is similar to the Viscosity software for osx/windoze, which uses the openvpn source code and adds the google-authenticator two-factor-authentication functionality. Unfortunately Viscosity, although based on openvpn, is closed source.