You might be thinking in terms of closed desktop data files like OLE.
Considering that SVG is XML, any external file should/can be handled as
a xml/html link. You can do that now easily in SVG. But another major
reason to include objects in your file is the "id" which allows the use
of DOM operations, etc though you could do that with the link on the
entire linked file (and maybe you can reference the id of the linked
file - I've never tried that). I think your linked files can be saved
all in one as a ".svgz" zip file.