How about a flight attendant asking for access and if no confirmation from the flight crew and an OK from air traffic control, a small door within the cockpit door could open so the Flight attendant could at least see inside and talk to the flight crew.
All of those carry the same risk. Hijackers can make a flight attendant tell air traffic controllers anything. What would work is an always-on video feed from the cockpit to galley — and a policy that any adult passenger is allowed to take at least a brief look at it upon request to a flight attendant, except when required to be seated with belt fastened due to take-off, landing, or pilot request over the PA system (i.e. due to turbulence). We need to take away the security pilots have (greatly enhanced now, with the locked cockpit requirement) that nobody can see what they’re doing.