The NS-2, and indeed several other noise control pedals, do half of what I've described. That is, you can plug into them at the start of your pedal chain, where the signal has the greatest ability to discriminate between signal and whatever noise exists at that point using amplitude as the basis for discrimination
, but apply the gating action at the point where the signal returns from the loop.
Here is the schematic: http://www.godiksennet.com/images/sch/NS2.jpg
You will see that Q1 forms the input buffer, whose output is fed to the output driver for the SEND jack (provided by Q2). If nothing is plugged into the RETURN jack, then the Q1 input buffer is fed directly to the RETURN point.
IC2a sets how much drive there is for the rectifier circuit, and IC3a/3b/4a/4b form the rectifier circuit and the part used for setting the decay time once the signal has fallen below the required threshold. IC1 is a seemingly proprietary dual VCA chip, only one half of which is used in this circuit.
So, to repeat, the envelope and triggering of the gate is derived from the input signal, but can be applied to either that same signal, or to the signal occurring at the end of whatever you insert into the send/receive loop. So, you can stick the NS-2 at the end of a pedal chain like a lot of people do, or you can stick it at the beginning but apply the noise reduction somewhere else.
What I have been advocating is a divide-and-conquer strategy, where you intervene at a point where a particular form of noise can be addressed most effectively in the least invasive manner. In a perfect world, I suppose, the first half would be a signal-controlled highpass filter that would restore bass when you play but roll off bass at a gradually higher point as you stop playing, so that hum would not find its way into your pedals. And with the greatest risk of hiss coming from the accumulated sizzles over all your pedals, the second half would apply a sliding lowpass filter to the final output, such that once your signal level drops below some point more and more of the top end would be gently rolled off. The LM1894 noise filtering chip was designed specifically for that function. While it is a dual channel chip, unfortunately, as near as I can tell, it was designed only for lowpass filtering and not to be reconfigured for something like dynamic hum filtering.
The 1894 is something we have collectively overlooked here. It's a nice chip for single-ended noise-reduction. It still seems to be widely available, even if not in production. Perhaps we could put it to some use.