The first attempt at a new dialplan application that can place free US calls through Google Voice has been released on the sipsorcery application servers. See this post Forum Post for more information.
The first question that will come up is whether it’s possible to establish the call without having to hangup the originating one. The answer is yes and it should be easy enough to bridge the incoming call from Google Voice with the originating one. I’ll have a look at that over the next few days as time permits.
Update: The Google Voice app has now been updated to avoid the need to disconnect the call, see this post.