What about those detectors with the built-in emergency light? I presume
that's just a flashlight type bulb (not an LED light) and if it can drive a
bulb it could drive a small reed relay. Wiring a relay in place of the
bulb is a pretty minimal alteration.

Rather than fooling with the noisemaker in each detector you could buy such
things (Sonalert or whatever) and place them next to each detector or
wherever you feel like so that there is good audio coverage around the
house (best place for a detector isn't necessarily the best place for the
alarm horns). Run Class 2 3 wire cable around the house. One wire is the
feed from some central power source...a battery...a DC supply or a doorbell
tranformer (depending on the needs of your alarm horns)...feeding the
relays in each detector. 2nd wire is what the other side of each relay is
connected to and is also what the horns are connecte to. 3rd wire is
common return.

Now when any detector goes off the alarm horns everywhere go off though
they won't be the ones inside the other detectors. You could wire them up
so they are the horns that sound but I figure it's best to leave them be so
that if your whole system fails for some reason (short circuit) at least
the one detector nearest the smoke will still sound even if the others
don't.