Opps, I never scrolled up beyond your last post ... stupid me.
Nice solution ... or was until they changed everything.
I'm back on 3.3 but when I had 3,4 installed I looked at the new structure, It's still all there, the categories have their own json file with each cat getting a unique id.
The main json.gzip (could be json.gz, I cant remember) file has all the assets data each with their own unique id which references the category id and the corresponding jpg thumbnail.
You'll need to unpack the compressed json file to see its contents, I think I used winrar but I can't remember.