Booting a diskette image via GRUB

I have a bootable diskette (for Spinrite, as it happens) whose image I've copied to my hard drive. I would like to boot that image via GRUB. I tried using the same method that's used to boot memtest86, but I get the message "Invalid or unsupported executable format". (I cloned the GRUB entry for memtest86 and changed the filename.) So what do I need to do in order to get the boot to work? I'm also interested in doing the same thing with ISO images of CDs. Paul