In a unix system, there is only one filesystem. Meaning: if you have several disks or partitions, either they are mounted in The Unique Filesystem, or they are not mounted and are just a blob of some gigabytes. The working directory is a per-process property, not per-filesystem. (except that windows has a current directory per drive letter, IIRC���)
In the case of memory filesystems, since they are useful for temporary stuff or for testing things on files without side effects on disk, you could see them as a sandbox where any starting point is acceptable. Or maybe we need a concept of a use session for a filesystem, and then the working directory is a property initialized when the use session is instantiated.