Current-sense for NES/SNES gamepad detection - getting a voltage spike
It seems a lot of trouble to go to when quite possibly a single resistor might work. The data output from the CD4012D shift register in the game pad will offer an impedance to the data line when it gets plugged in. 

Based on this "theory", if you had a pull-up resistor in your detection module (the bit you are adding) then when the game pad is not inserted the data line will be high and, when the game pad is inserted the line will be low or, show low approximately 50% of the time it is producing data.

If you are unsure about data being high or data being low then two resistors forming a midpoint between logic rails should work - without the game pad connected, the voltage will be 2.5 V (assuming 5 V logic), and with the game pad connected, the voltage will be 0 V or 5 V.

Maybe I'm missing some subtlety?