What I want to do is present a directory, the contents of which are sourced from a series of other directories on the file system. It would be like a soft-link, only a link to multiple directories at once, rather than just one.The directory might be considered read-only - or, if updates to a file are made, it would be good if those updates occurred in the source-file (i.e. the directory is a window into a number of locations on the file-system)If a new file is created in this 'pseudo' directory, it should either be dropped into some default location, or rejected altogether.Similar functionality exists in Windows (under the name libraries) and OSX (smart folders) - is there any Unix equivalent?
Unix equivalent of smart folders
filesystems;symlink;directory structure
It's not available as part of standard Unix or a graphical Linux interface.Linux system administrators can use overlayfs.  Actually one of the most important uses is to allow modifications to a running LiveCD system, e.g. installing extra packages.There are also equivalents in FUSE, which can be used on Linux without root privileges.  There will be additional limitations from FUSE but it could work well for the same cases as the features you mentioned.  unionfs-fuse appears to be available in ubuntu.  (union mount is an older term for the concept).