Welcome! As a musician myself, I can appreciate your desire for music on the trail. Waterproof breathable materials work when there is a temperature and/or humidity difference on either side of the fabric. (Just to note, I'm a little far out of my league scientifically speaking, so forgive me if I misspeak--others will correct me if I do ! :) )
Basically, there needs to be a reason for the moisture inside such a case to be transferred outside. Since a guitar is an inanimate object, it does not produce heat or humidity, and waterproof breathable materials would not be of much help.
Moisture for wooden instruments is relatively good. Dryness, extreme cold, and extreme heat are what you want to avoid. My suggestion is to have a lightweight hygrometer inside the case. If it is too dry, then add moisture. Products are made for "hydrating" an instrument. Look at Planet Waves Guitar Humidifier. If it is too moist, then maybe consider packing a lightweight and reusable desiccant. Check out the link below.
My suggestion would be to get/make a padded waterproof case with a waterproof zipper. Unless you're dumping water into it, with a little care, your guitar should be fine.
I'm a violinist. My instrument loves humidity and hates dryness. Just be sure to monitor the proper humidity levels in your case. But then again, you're on the trail, so...... :)