I couldn't find anything on the internet, the closest was feeding rice straw to ruminants. I can conceive of mushroom waste as silage without the bacteria, but I don't see it as a whole
enough food. There is good hay and bad hay, I don't see it as great silage but a reasonable binding and bulking agent.
If I had some I would definitely like to try it, rabbits are mini ruminants there just not as grand on eating gruel forage perpetually without getting ill.