I've done a project with OctoPi customization before but I don't see any relationship between smoke detector and 3D printing (fyi... my previous project is adding the automatic resume printing when power outage & enable user to specify the Z level).
Regarding to this project, my proposal is - instead of connecting the MQ8 sensor to the Raspberry PI, we will use the ESP8266 WiFi chip instead. The firmware on ESP8266 will be developed to read the output of MQ8 and if it exceed a predefined value, the relay will be turned on. With this solution, you just need ESP8266 ($3-$4), a relay with a transistor and resistor and of course the MQ8.
You don't need the ADC module and the total size is very small. My bid does not cover the custom PCB design for the solution.
This is also not include in my bid but if you want the OctoPi to get the relay ON/OFF information from ESP8266, we will add a web server inside esp8266 and octoPi will read the value via Rest API.
Any question, pls let me know.
Thanks,
Anh