Plan 9 came close. You could create /foo/a and /foo/b at your leisure, then bind /foo over /etc. This atomically replaced /etc/a and /etc/b with /foo/a and /f

Plan 9 came close. You could create /foo/a and /foo/b at your leisure, then bind /foo over /etc. This atomically replaced /etc/a and /etc/b with /foo/a and /foo/b, for the purposes of open. The change could be atomically reverted with unmount. If that wasn't good enough, then /etc/a and /etc/b could be virtual files served by the same daemon. This could look for change sets written to /dev/patch, and block reads and writes while it applied them.