hmmm Let me get this straight
I definitely understand that we're not talking about a single download.
A single download would just be downloading the entire ISO Image as any other file one could download over the Internet, like the ones offered at LinuxISO.com
I also know that jigdo uses this two special files (.jigdo and .template) as it's guide to assemble the Image, I don't certainly know all the details behind the structure of these two files but if they're meant to tell the jigdo engine how to assemble the Image and what files to grab, I can imagine they're setup in a way that they know the grand total of files the Image is made of.
If they know the list of files and the total files, Couldn't they be able to know or at lest find out the size in MB of each and every of these files?
If they could I don't see a reason why it would be logically impossible to calculate the %completed, plus the other stats I mentioned.