From a safety system perspective this kind of error simply should not happen. Maybe that's why so much media interest. It flies in the face of the attention to safety the industry purports.
As for fail-safe signage, firstly signage should not be the primary defence. Some kind of system lockout that prevents entering a flight plan that is inconsistent with the aircraft's certification is an obvious primary defence.
The most fail-safe signage system is to have signs indicating both ETOPS and NON-ETOPS using colour coding (can be black on white or white on black). No sign means pilot must regard the aircraft as NON-ETOPS and confirm. Although there should be a prior check which means an aircraft without a sign cannot go into service.
These things are not hard, they are safety 101.