Leave nothing behind means if you pack it in, you pack it out. Not sure if there's a substitute for TP other then leaves and pinecone. In certain area's of the Sierra's, it mandatory you pack out your used TP. Mt. Whitney supplies Wag bags for more then just TP, and that mandatory as well. Kinda hard to get past our old ways of just bury and forget, but it's for good reason.

You could try nettles or poisen oak...