Building large KML file

Question

I generate KML files which may have 50,000 placemarks or more, arranged in Folders based on a domain-specific grouping. The KML file uses custom images which are packed in to a KMZ file.

I'm looking to breakup the single KML file in to multiple files, partitioned based on the grouping, so rather than having 1 large document with folders, i'd have a root/index KML file with folders linking to the smaller KML files.

Is this possible though? I think that a KMZ file can contain only 1 KML file, regardless of where it's located or its name, in the zip. Furthermore, I'm not exactly sure how a KML file can link to another KML file. Is the only way to have it as a <NetworkLink> to a local file? Can a <NetworkLink> work to link to a file local in the same KMZ?