Unfortanly, we do not have a security framework like apparmor on the N900(probably selinux, but nobody cared about it yet afaik). Anyway, you need Access Control Lists (setfacl). There is just one problem: the rootfs is ubifs, and ubifs does not have acl support, which basically prevents/limits you to achieve what you want.

But there should be other/better solutions to stream music from the N900 than using sftp/sshfs or something like that.