That's because it can't be done this way in FileMaker.
You can do the opposite of what you describe so you may be able to get something figured out.
You can export a single PDF file from the container field and then use Save As PDF to generate a PDF and append it to the exported PDF.
No good, so far.... I can save the entire record (including the container) as a pdf, but I can't save an exported container that was already a pdf as a pdf. Kind of inefficient: exporting / appending pdfs-in-containers into existing pdfs would be a nice feature to have in the future.
I'm still looking for a good permanent solution, if you can think of one.
In case anybody else is in the same bind, I did create a bit of a jury-rigged solution that's a variation on Phil's suggestion:
I created a layout in which I displayed the contained object on a full legal sized page. (All my attachments, fortunately, are legal sized, else I'd need a much more complicated solution.)
In my script, I use that layout, exporting it as a .pdf with the "append" box checked, to the existing pdf at $$filepath. Needs tweaking, but at least gets the object in the .pdf report.
Not pretty, but it's doing the job, sort of. If anybody can think of a better work-around, I'd love to have it.