Is there a practical way to create/inject some visible interference into a nearby AC mains?
Any powerline communication device does exactly that: inject a signal on top of the power signal, and extract that at the other end. It works.

The fact that you don't see much with an oscilloscope just means you're using the wrong instrument.

Try the following: 


use a dimmer for resistive loads (for incandescent lamps, i.e. the cheapest dimmers you find).
set it to a medium dim
attach a large load, e.g. a hot plate, toaster, large lamp, whatever, close to the maximum rating of your dimmer
use the transformer, plugged into the outlet you want to observe, only as a source for the trigger of your oscilloscope
use that trigger signal to trigger the oscilloscope, observing a capacitive divider: two (high-voltage, medium to small value!!) capacitors of two different sizes in series between live and neutral of the power outlet you want to test.


What does a cheap dimmer do? Typically, it's a phase-fired controller, which means it always turns on the load (like you, flipping a switch) at the same phase (i.e. angle) of the grid voltage.

That's awesome, because a powerline system has different properties, depending on which phase you look at it.

Now, your transformer might not even have the bandwidth to notice the high-frequency noise caused by switching on, but your capacitive divider definitely has – in fact, it should act as kind of a high-pass filter.

By triggering the oscilloscope with your transformer, you make the grid phases take a "fixed position" on your oscilloscope screen. That's immensely helpful, since now your switching noise will also always get a fixed position (since the switching always happens at the same phase), and be much easier to spot. 

Compare load with and without dimmer, and with no load and dimmer at all. Then start playing around with the dimming ratio.







simulate this circuit &ndash; Schematic created using CircuitLab