You could use fuse for this. Just have a script to handle the /mnt/extlibs mount point and let it take you url as the argument. I can't see this being too difficult. Indeed the /mnt/extlibs could just be installed as part of the library search path. You could then do:
import http/www.google.com/code/widgets as widgets
and when the search got to the /mnt/extlibs it would see the http prefix (or whatever convention you used) and get the code.
Works on Linux and Mac and would probably work for Perl and other languages as well.