Mirkwood logs seem like an ideal little container to have a simplistic system for fungus to be cultivated in. It could be as simple as placing a fungal "seed" in the log which produces the forageable and occasionally a new "seed" over time. Possibly a mirk log with an active fungal colony could have a little mushroom growing on the log visually. This would allow for some degree of quality increase in a similar manner to farming.
Rare secondary spawn when picking mushroom in the wild could spawn a "seed". Oospore would probably be the most accurate term since those fungal spore masses can hibernate, but for game-iness sake fertile mycelium would work and maybe more recognizable to players.
Just a little idea, well aware it'll likely never see the light of day but that's ok.