Now, by "copying" I meant to do something like this:

    # cp /packages/emacs-21 /stow

    that is, to _copy_ the package into /stow (that is, having a real directory
    into /stow instead of making a symlink). This was only the main way I used to
    test the stowfs support (is more interesting on a debugging POV since files
    are more slowly added into the union and I could test better the support for
    dynamic insertion of directories). _BUT_ symlink are supported too, that is we
    _can_ install emacs into the system just by doing:

    # ln -s /packages/emacs-21 /stow/

Ok, now I understand.  This is the right thing.