Is this possible? Can an app make this happen or is there a physical hack I need to incorporate?
Interesting question but I don't think it is possible.
The physical hardware doesn't exist within the device. This would be an expensive physical hack. You might be able to access this feature through the dock but you would have to create another device that basically can shake your entire tablet.
There's no motor inside the Nexus 7 like in cell phones, so it doesn't vibrate.