Depending upon how the smoke detector module works, different approaches could be done. If it can handle the current, it might make sense to run all of the power for the printer through it. It would just cut power (and maybe later reset) if it detected smoke. Marlin will come up with the motors stopped and the heaters turned off.
You could also run a wire to the AVR's reset pin such that the smoke detector could reset Marlin if it detected smoke. But most likely, it would be best not to have the firmware in the loop making decisions.
