I'm not sure if this has been suggested already, or if Obi is already working on this, but the following feature would seem a relatively straight forward way of providing GV dial out support when used with a third party dial-in number. Dialing a number on the handset from your GV number seems to be the main loss from the GV changes.
What's needed is a feature that lets you log into your GV account and trigger a call using HTTP via your Obi box when you dial a number on the handset. The dial out would appear to be normal but the box would instead trigger a GV callback and be in a mode that accepts incoming calls on your dial in number from your GV number. The Obi box answers the call when it arrives, automatically connecting you to the dialed number.
Besides being a bit of a hack, and some usability issues, I can't see why this wouldn't work.
