How can I use the presence or non-presence of an 120VAC input to actively discharge a bias power rail?
The circuit you've shown (assuming there is 120VAC connected to an unseen bridge rectifier with the '-' connected to circuit common) 'looks' like it should work. Obviously those diodes cannot actually be 1N4148s since they're rated at only about 75V.

The time constant of R2/C1 is less than 10msec so there is no way it should take seconds to discharge (even if you allow for the gate capacitance on the MOSFET- presumably you're not actually using that part number, but something much smaller anyway). If there was enough leakage from the main supply capacitor through the main bridge rectifier that could  explain it (so the problem would be 'off the page'). 

Perhaps if you split the (off the page) bleeder resistor for the input filter into two resistors, each to the common, rather than one resistor across the line..