Those 150 or so plant nutrients said to be present in good quality castings must obviously be supplied by the worms' human caretakers, either in simple elemental or complex molecular form.
Would feeding already rich mushroom compost -- already brimming with organic goodies -- to my earthworm bin produce an even more rich "super" vermicompost? How about adding kelp meal, which should ensure a complete dose of all necessary secondary and micro nutrients?