I think the cap will be ineffective.
I know what you mean, but it's not that bad.You can have a smoother LED current if you also add that 220ohm resistor between bridge and LED (post#17).Leo..
The function of LED B is to set a digitalRead pin HIGH.
What about using a 0.047uF 250V capacitor in place of the two resistors? In AC circuits, capacitors supply impedence (0.047uF at 60Hz = 56kohms) but they don't dissipate or generate heat like the resistors do.