I think the idea for a decal OR code may have some good applications, just as I like the large, laser-etched codes on the signs shown above. Some aspects of the decals used for signs already are that they're durable, there's an established size and format for the decals and they can be simply replaced by covering over, as one does with the registration decal on your license plate...
There's a carsonite sign.
One cool thing about QR codes is that they don't have to be very large. One could use them in a system with laser-etched trail signs and one could use smaller tags, even down to the size of a postage stamp, on signs, in trailhead kiosks, in trail brochures, etc... and they might function to reduce the number, size and expense of trail signeage, while providing a low-impact, high-reward experience for those seeking information ranging from interpretive highlights to positioning or map data.
One could create a custom app for the particular location or region, or one could simply code URL's to "good old fashioned web pages" or even a GoogleMaps location.
Those of us offended by technology can ignore the organic-looking symbols and savor the fact that they might be the only alternative to large, buzzing neon lights with solar-powered, action-video kiosks... The sky's not falling and the outback isn't being made any more accessible or friendly to those soft ones who haven't walked uphill both ways with torn, free-range soy birkenstocks or 40lb of rocks in their pack. It's gonna be okay. :) It's just an idea.