My point was that the metadata abilities of HFS+ are not relevant to the question.
I have just created a Disk Image, partitioned the image using MBR and formatted the image using MS-DOS (FAT16). I then added a couple of PDFs files to the disk image and used "mdutil" to turn on indexing on this file system and to erase the spotlight index and thus force a reindex of the drive.
Subsequent spotlight searches when this disk image is mounted returns search hits based on the content of the PDFs. (The search was "The first 13 states" since the pdf documents were about US History.) When the image is not mounted, the spotlight searches do not return the files stored on the disk image.