Well, Straw and Hay are staples of mycelium growth so taht part is covered.
I have seen people use egg shell for design objects in Colombia so I know is possible, likely to be eggshell pieces binded by polyacrelate resin. It all calcium carbonate in the end.
the chemist in me says:
Grind your egg shell to a fine powder and use them durng the drying process of your Kombucha leather. Since a main complaint in Kombucha leather handling is the humidity and carefulness required during the drying, it might lead to somethin useful.
I dont know if you can do waste coffee Kombucha, likely the tannins are too high, check with @niekd.