Not even the kernel knows how much space is available on a nonmounted partition. It doesn't know much beyond the fact that it exists and where it exists. There exist tools that can analyze nonmounted partitions, but these will vary by filesystem and operating system. If someone wants to implement solutions for the most common filesystem-OS combinations, then that might be helpful.

But considering that no one has done that or even asked for it yet, I'm not sure that doing so is worth the effort. So I agree that documenting it is the best approach. It should be documented such that the limitations are mentioned, not necessarily how it's implemented. If explaining which system calls are used helps explain the limitations, then so be it. Additionally, as far as I'm concerned, there's no reason to restrict other Python implementations from implementing functionality for nonmounted disks. So the limitations should be described as being CPython-specific, akin to what is done for `readline.set_auto_history`.