Following up to [1], I have implemented the stuff I mentioned was missing.

  I realized eventually that there is no way we can make the namespace and file
name be the same, therefore I added a new 'shared_library' string to the
metadata file.  It is optional, but it can point to the shared library that
implementes the functions described in the metadata.

  Moreover, I created a GMetadata structure and replaced most occurrences of
"guchar *metadata" with "GMetadata *".  GMetadata holds the metadata, but can
also hold a GMappedFile and, most importantly, a GModule.  Since the GIBaseInfo
was changed to keep a pointer to GMetadata, g_function_info_invoke now uses
g_module_symbol to lookup the symbol from the correct library, whose symbols can
therefore be loaded as private symbols, and lazily.

  And yes, I practically gave it away already, memory mapping metadata is
implemented already.  And so is a metadata searching.  There's a "search path",
with a default value $(libdir), plus the colon-separated directories listed in
GIREPOPATH.

  Also the patch fixes some build/configure problems, makes the 'invoke' test
actually exectute on make check, and adds a new test to check the new
namespace->file mapping and automatic module loading.

[1] http://mail.gnome.org/archives/gtk-devel-list/2005-August/msg00006.html