As a point of interest, I recently suggested a much similar approach in a post on another forum at pprune. The bottomline concept - to make success impossible for a repeat of the murderous WTC strategy - is important as a deterrent to future attempts; the final technical details of implementation are important and must be left to appropriate experts, but the functional objective of preventing takeover kamikazi missions is clearly essential - and it is clearly achievable with current aircraft using readily available reliable technology.
FWIW, my 091201 post to pprune in re same topic:
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Controlling hijackers is a problem with onion-like layers. Although the possibility of a disaster like the WTC takeovers has long been a danger that risk analysts could forsee, the financial and political motivation to put teeth behind its prevention have been lacking. The DB Cooper model of 'benign' hijacking seems to have prevailed. Now we clearly perceive a broader and deeper threat that requires more powerful counter layers.
At the top level, Control Access - to the aircraft, to weapons, especially to the cockpit. The classic formula.
In the middle levels, Limit Ability of pax to take control of the cabin - via shotgun guards, mace, karate-trained stews, thin air, lighting, etc.
At the next level, Limit Ability of any hijacker in control of the cabin to enter the cockpit - with physical barriers, crew weapons, and other kinds of gotchas.
At the final level, Eliminate the possibility of completing the hijacking mission - at least the WTC/Pentagon style and preferably most others types - by auto limiting control / course/ range/ speed options when authorized crew not at controls, by including automated squawk functions when specific crew members absent or inop, and maybe for FBW aircraft something along the lines of a deadman's control that limits power in hijack case as soon as aircraft is within x-range of a suitable airport, perhaps triggered by combination of inop crew and ground signals or chase plane telemetry.
Desperate measures, but more benign and therefore more likely to be invoked than shooting a wayward airliner out of the sky. All the above - and more - relatively cheap with contemporary technology - somewhat analogous to an ejection seat - powerful stuff, not for casual use.
The ultimate objective is to convince methodical would-be hijackers, before the fact, that they cannot successfully achieve targeted kamikazee style mission objectives.