I'm not really sure what you want to do, but here are a few options:

If you really want to have textures accessible from different paths without duplicating files, then you'd have to use hard links. But those are poorly supported by Windows, so I wouldn't suggest to do that. Would work with any application  that accesses the file system though, so you can use whatever you want to browse your library.
You can also use regular file shortcuts, but those will only work for library files and not geometry or image files. You'll be restricted to applications that can deal with shortcuts to browse your library though. Poser can deal with those for library files.
You can create a "virtual library" with Advanced Library. You'd be limited to browsing that virtual library with AL then, but no change to the file system would be needed. Of course Poser can't see the virtual library then, but you can use PRPC to load the files into Poser.