Only problem with LF stuff is that you need a pretty big antenna. You would be better off designing a chip with an RF section that operates in the GHz range so you can get away with a tiny microstrip antenna or with the antenna inside the chip.
I will admit radio is not my thing, that works too though and then you can make it a much larger activation key, even just bytes instead of bits would exponentially increase odds of not having an accidental activation which should make it not get set off by a router or microwave and you could probably activate it simply by flying over with a slightly modified cargo or passenger aircraft that looks business as normal but just belches high-power bursts of the activation signal.
