we were discussing this on the IRC. Imagine spinning up a thread, then running a bsd style pledge(2) on it to call liblzma. Kinda janky but it would work. Another option would be to just go out and call the xz util and not rely on a library to do so. That process can be locked down with pledge to only have stdin/stdout. That's all you need.

So, like UNIX does have this plumbing, its just that reaching for libraries and tight integration has been the pursuit of Lennart Poopering and his clan for years.