I'm going to be getting a couple of induction hobs, but I am wanting them mounted underneath a thin (approx 5mm) piece of wood. Is this possible from the safety point of view and would I have a major worry about efficiency?
Induction hobs work by creating a magnetic field which causes electrical currents in the base of the pot or pan. These currents heat the pan through normal electrical resistance. The further away the pot is from the hob, the weaker the magnetic field, so you're going to lose a lot of efficiency right there.
From a safety point of view, there are several oils that have smoke points over 400 °F so if you ever fry your food, you'll probably start to see the wood charring.
Finally, how are you going to keep the wood clean? Any spills are just going to soak right into it.