Google Voice "Call With..."
There's an app I used to use specifically for this. It's called Voice+. You dial a number, it asks you which of your google voice connected numbers to call.

However, this app catches the intent from when you dial a phone number. The Nexus 7 doesn't have the Phone app installed. You could try installing another dialer app like Go Contacts Ex and see if dialing a number from their interface brings up Voice+.