I've been working this issue myself. I think i'm in the home stretch but am hitting a snag...
So, I got all the extras via the python script https://github.com/carlashley/appleLoops. Ive got them all neatly sewn up into a single installer.
I Install Garageband via the app store, then prior to launching the app, I install the extras. Garage band launches asking to download content. I skip the install and go into the program I see the extras as available. But each time I subsequently launch GB it asks to download content again... It seems stuck in a loop (see what I did there? a loop... getting the loops for GB, I get a loop) I cannot get the admin prompt to go away nor make the stupid thing realize its got everything it needs to run. Ive tried re-indexing with no luck
Anyone have the receipt, plist, defaults write, combination of chicken blood and grave dirt, or string of expletives that might make the loop, loop go away?
Also, anyone have luck with https://github.com/jacobsalmela/adminscripts/blob/master/downloadGarageBandContentPkgs.sh#L351-L426 (the bash version of the python script above)?