Would require major redesign of lavs, etc.

I think the reinforced doors are sufficient. I can think of two
deterrents against hijackers:

1. Violent manuevers. Warn that everyone should strap in, if the
hijackers don't they will get tossed about.

2. Quit pressurizing the main cabin. You want O2? Sit in your seat
and grab a mask. This would only work above, say, 15K feet. Also I
understand there are portable bottles for the FAs, still it might
work.

Obviously these are last resort, but with the reinforced doors
nowadays to slow down attackers, the above could gain some advantage.