I'm not sure what that 0.33uF (which, btw, is 330nF or 330,000pF) cap is for exactly. My first reaction was that it was for shock protection, but the only time I've seen those is when they are parallel with a resistor. Either way, the ground and shielding are not completely separated, they just have that extra component between them. Connecting the ground directly to the shielding is really the only way shielding even works. Connecting the bridge to ground reduces hum when you are touching the strings. However, as you have pointed out, with nylon coated strings, it may not work (I'm sure it depends on how thick the coating is; for instance, I've never witnessed Elixir strings affecting noise reduction due to their coating).
You could always consider piezo pickups for a fretless if you have yet to buy pickups.
EDIT: Maybe the capacitor effects the frequency of the noise being introduced into the circuit, sort of like a tone pot for the hum? Just a though, and probably completely wrong.