I finally finished my connected smoke alarm project this evening when Kidde relay arrived. Thanks to all the posters who shared their project!
I did not have an easy way to install a junction box, which is the main reason I delayed in doing this. Then last week I had a brain wave to try and see if I can use a broken smoke alarm I had laying around. The result is in the picture below 
. I used a Peq open/close sensor, which seem to be exactly the same as the SmartThings sensor. I de-soldered the reed switch from the sensor and soldered on three wires, which connected to the Kidde relay common, NC and NO. So it’s not quite as simple as the Schlage that folks were using.
I had to Dremel everything out of the donor smoke detector and it was still a pretty tight fit. I can probably free up a little space by shortening the wires from the Kidde relay. But for the moment everything is in there and working. I replaced the original smoke detector connector with a re-purposed PC hard drive power connector male / female set, so I can unplug it from the ceiling for future battery changes or if I want to move it.
As noted in other posts, I changed the device code to make it a smoke detector device. I also rewrote a bit of the device code to emit smoke events instead of open / close events. This allows me to integrate perfectly in the Damage & Danger section in the dashboard, and add rules for being notified, turning on lights, unlocking doors, etc.
I don’t know why one of the alarm manufactures don’t come out with a hardwired alarm that you can connect into the existing interconnect wires. E.g. if the wired Nest allowed connection and monitoring of the interconnect wiring, I would not have bothered to go to all this trouble.
