Used mycelium substrate is a GREAT composter! When I was taking a Master Gardener course, as soon as people found out I want to grow mushrooms many of them were asking for my spent substrate.
I haven't fed substrate to worms yet, but as I build up my mushroom and worm production that is my plan. From what I've read it is great for worms.
I've also heard of spent substrate fed to worms actually growing mushrooms, which can also be used as worm food. I'm excited about trying!