I had the exact same thought myself back in 2008, so I asked an experimental psych professor about this. At the same, he said that the TMS devices that we had are somewhat wide-area and also induce considerable muscle activation. This doesn't matter very much when studying the occipital lobe, but for the prefrontal cortex you basically start scrunching up the person's face, which is fairly distracting. Maybe worth trying anyway.

I've wanted to get my hands on a TMS device for years. Building one at home does not seem particularly feasible, and the magnetism involved is probably dangerous for nearby metal/electronics...