How does FrameMaker handle text insets in XML ?

I am working on a large XML application and now need to use inclusions (referenced files). All files are in a custom XML format. I know FM can handle these types of inclusions as they do work for DITA content, but I cannot figure out the read-write rules I need to make it work on my custom XML. It looks like the dita_fm client does all kinds of things while importing and exporting, but that is not documented anywhere.

I can create a script that pulls the included files in after opening the main document, but when I write the main document to XML I do not want FM to push all the content of the included files into the main XML. I tried using a rule (which is described in the read-write rules reference) 'writer drop content' but that rule seems to make no impression at all.

Anyone has experience with custom XML inclusions ?