The output will be digital so your comparator option looks right. If I don't control the output what will it be? Lets say I set it to switch at .6 volt input, what would the output be? Would the output be the same as the set point?
The reason I'm doing it differently is that the typical circuits offset the voltage a bit so the ECU sees .6 volts or so when the O2 sensor is reading .5 volts so the ECU will lean out the fuel a bit as it thinks the exhaust is richer than it is. When you add fuel supplements like propane, water injection, hydrogen, etc the exhaust normally appears leaner to the ECU so it adds fuel killing your economy. The circuit I'm considering should work as well or better than the others because the ECU is only looking for a digital on/off signal anyway and it should be easier to alter the trigger point for the output with the comparator circuit. The other circuits seem to have stability issues as well and a difference of .1 volt can make a difference.