[anitil]

Wow this comment just made me fall down a rabbit hole. I've only just surfaced. The Kaitai project actually comes with some pre-defined bindings for cpio which meant I was up and running very quickly.

https://formats.kaitai.io/cpio_old_le/index.html

[topspin]

Yes, cpio is ancient and simple and very easy to work with. The Kaitai project didn't exist at the time; I used the C structs documented in man pages.