How about a double door where only one door can be open at a time (like photography darkrooms) to a narrow passage that requires you to enter sideways (watch your waistline) that can only hold one person at a time? This would stop hijackers from bursting in with the next sharp object held against a hostage's throat. The cockpit could have a switch to open both doors on the ground for evacuation and ground servicing. Once pasengers are boarding, the passage would revert to secure mode.
Additionally the galleys must be protected so that one or more hijackers can not quietly sneak up to an isolated cabin crew member to make a hostage of her. It's much easier to make the initial hostage grab in the galley where there are no passengers to interfere. These people tend to do their dirty work in dark corners.