This seems like something that should be built into the Google Voice app. For my usage, it should remember which contacts should be dialed with GV and default to non GV for all the rest.
I did find an app that adds some of this missing functionality:
Voice Plus. It lets you select which numbers you want to dial without GV. To me, this is backwards: I want to select which numbers I want to dial with GV, not the opposite. Also, for functionality that I think should be built into the GV app, the price point for this app seems a bit steep at $5.99 (but I would buy it if it could work in this opposite manner).
Steve