That should work currently, because this falls into the "listing for paths which match one of the registered filesystems" case.If I have a use case for only one file system and want to mount it at "/" then it would then seem to be desirable to be able to perform a directory listing at "/".
However if you register a file system with "/sdcard" prefix, and try to perform opendir("/"), that will not work, in the sense that it will not list "sdcard" as a sub-directory.