Optocoupler in reverse-active mode
The photo transistors used in most optocouplers have a very large collector-base area, so that the light from the LED will have the most possible effect on that junction.  Operating the transistor in the inverse mode (emitter positive, collector negative) should work slightly, but the reversed biased junction is now the emitter-base junction which has a much smaller area, thus not nearly as sensitive to light.  I'm having trouble seeing what benefit you might expect from this mode; perhaps it could be faster switching (but there are better ways to do that); perhaps lower saturation Voltage.