Ok, I'm going through the english manual and noticed that in Part 2 of the manual on page 3 (real manual page 43, Japanese manual page 40) there is a screenshot of how to run a game off of the memory stick.
The name of the game is "Big Bang Bang" and some of the thumbnails shown match the ones pulled from the "network update" rom file.
Perhaps just that picture could be used to help decode the file a little more since there seems to be more than just that one thumbnail is shown.
I was actually thinking something along those lines. I can't count the number of times I've written code just so that i can show someone "code" in a screen shot or a small demo program to be called by the "real" program.