I'm pretty sure it's expanded paper. I used to work in the garage door industry, and they used it in hollow core masonite doors all the time. It's also in hollow core doors for your home. If you look real close sometimes, you can see that the matrix isn't perfect, sometimes it sticks or tears and comes out kinda funny looking. Here is Rocco Passionino's picture of the cargo bay wall, the original is to big to post here, but here is the link:http://www.roccopassionino.com/Images/photos/2002/2002.03.01.Firefly/DSCN0306.JPG
In the upper right you can see some paint from the cargo bay "dividers". You can also see some "weirdness" in the matrix. It comes in quite a few sizes, the stuff we worked with in the garage door industry was one inch thick. The holes were about the same width, give or take just a smidge. You can also see this stuff in the passenger lounge, sick bay, cockpit, the galley lounge, aft hallway, engine room, and of course, the crew quarters. It's my opinion that it's all the same stuff. I think they painted the plywood set walls grey, and sprayed glued on the walls and left the expanded paper it's natural color (which is a card board brown-ish color).
I was recently at a wedding reception. In the Reception Hall's closet there was a table that was damaged. You could see expanded paper inside the table. It was about half to three quarters of an inch thick. The stuff is everywhere if you're looking for it.