if the fire alarm is in a different room light would not work very well, with a strong enough smell no matter where you are in your dwelling you are going to notice it...
I don't see how smell wouldn't suffer from the same issue as light.
If you're in another room (behind a closed door, for example) I can't imagine it's tremendously effective
I'd hook the smell-alarm up to the ventilation system, and wire it to turn all of the ventilation fans on if the alarm needs to go off.
Isn't that going to spread smoke around and make the fire more dangerous? It would be better to just have smell emitters in every room.
There are already phones for deaf people that you can rig to your house's light. When the phone ring all the lights of the house start flashing.