I've got a couple true life examples of alleged KGC (or whomever these folks were) code somewhere in my boxes of old stuff - I'll see if I can find them and scan them. One is a Baconian coded letter given to me by my good old deceased mentor JW from Kansas - I can't remember the content offhand, but Baconian code is an interesting topic and a good way to hide things in plain sight.
The other is a nice fit to the idea of so-called "persher code" used by Poe and others - information hidden in the form of a narrative containing certain surnames, place names and/or a series of events that privileged folks recognize as important to them for some reason, but that the general public reads as ho-hum. Two meanings - one for those "in the know." This example is a local old newspaper article - an innocuous report of mining people in town to inspect a small up and coming mining district. These types of things were commonly found in the papers of the day in mining country. In this particular case, the article mentions a very minor, obscure and remote bunch of claims that, IMO, is coincidentally the exact site of an alleged coin meltdown operation following the 1933 Gold Act. I don't believe in coincidences, and although this alleged activity cannot be "documented" to Ol' Kentuck's standards, I can provide a mountain of circumstantial evidence that would likely arouse most peoples' curiosity.