if airlines can link their entertainment to their PA to override IFE audio with announcements, presumably it wouldn't be too much harder to rig the noise reduction system with a similar override.
as for senior fd crew being used as leverage by bad guys - surely the principle should be that once the cabin has been compromised the door should not open until the aircraft is on the ground on any pretext? Even if not, if the relief crew can't handle a situation of this kind adequately it calls the use of such crew into question except in situations where crew rest is in the secure area and thus the commander remains available to take charge.