With exiftool 6.93 and earlier, XMP from the main document as well
as from embedded images as long as they aren't encoded. There is
no controlling this, and all the XMP will be mixed together. I don't
have a PDF with an embedded image such as this so I can't test it
out myself. If you could send me one I could take a look at this
(email it to phil at owl.phy.queensu.ca -- up to 10 MB should be OK).