Over a month ago, I was on a short trek in the jungle heading back to Juara beach on Tioman Island after spending the afternoon at a waterfall in the jungle. As I walked, I spotted a polypore that had been broken off a log and placed on top of it, perhaps by a passing group of hikers. Some of the shelf mushrooms that have very fine pale pore surfaces can be used to write messages. The most famous example of a fungus with this characteristic is Ganoderma applanatum, known as the Artist's Conk. I'm not sure what species this example is.
Of note: many of these larger, very solid polypores can continue to release spores for seasons, so please resist the urge to break them off the substrate.
I scribbled a message on the underside, leaving this diminutive billboard with one of the smallest possible audiences--hikers with good eyes taking the difficult route to a particular waterfall off the less-trafficked beach of a small Malaysian island in the South China Sea.
Today I received an email from a vertebrate/invertebrate researcher with this picture attached. Not exactly guerrilla marketing, but probably about as close as I'm going to get.
(Thanks for the email, Reuben.)
[Currently working on a more detailed post about contract mushroom farming in developing countries, sorry for the delay in posting]