Mono,
The system on the 757/767 is obviously different to the 747 system. In the 747 system the bleed air is not in itself passed through the sensors for obvious reasons which you refer to. Instead the flow of the bleed air is used to create a vacuum which draws in the air from the cargo compartment through the sampling ports. It is this air which the detectors check.
From what you say it sounds as if the system on the 757/767 is better than on the 747 as at least you have some form of redundancy.
So what would you do Mono if you had a twin fan failure while making your way across the Atlantic / Pacific ? And come to think of it what about a failure of your engine fire detectors if you're on an ETOPS sector, probably a more critical failure ?