Well done. I've installed 3 jam packs now (1,3 and 4) and you're right - the loops and instruments are now stored all over the place. And they take up a huge amount of space. So like you, I've wanted to get them off my boot drive and on to an external but I just couldn't be bothered trying to hunt them all down.
I'm surprised you didn't find an article on the Apple support site since it's a very popular topic. Most people have done it using a symbolic link.
Anyhow, glad you made it. Now if I could just do it myself...