Actually, I'd like a different approach that's effectively immune to hostage-taking.

Put some GPS systems on the planes linked to a database of the surface of the Earth. This would show the minimum safe altitude everywhere and it would show where a plane can land.

In it's normal operating mode it would simply act as a warning to help prevent controlled flight into terrain situations. (There have been accidents this would have avoided, that's probably enough to justify it right there.) If the projected flight path is going to bust the minimum safe altitude it starts yelping at you.

It has two more modes, though.

In the first hijack mode it simply will not permit busting the minimum altitude. The computer takes over the controls to the extent necessary to ensure this. It's data also shows habitation, it will turn away from dense areas when the fuel gets low enough. (It knows where the airports are, it will permit you to head for one.)

In the second hijack mode it's far more aggressive. It knows where there are airports that it can autoland, it heads for the nearest unless that airport is broadcasting a waveoff (at which point it selects the next nearest).

The controls still work but only to maneuver within a certain range of the flight path the computer wants.

Once set you get a few seconds of a squawker and then you can't turn off the hijack setting--that requires a ground crew.

Breaking the cockpit door also sets mode 1 automatically.