The one thing that Garageband does differently than any other host on the entire planet is to apply Sandboxing which means that the plugins will not be able to access the normal filesystem but use a container just like on iOS ("Every App is an island") as root file. There were rumours about Apple applying that to Logic in some time in the future, but it's unlikely to happen unless they want to jeopardize their professional DAW and turn it into a bullshit app like GarageBand.
The only exception (don't ask why) is the ~/Music folder (which is why the sample location defaults to this place). Now there is a preprocessor macro in HISE called ENABLE_APPLE_SANDBOX which moves the entire app data folder from the ~/Library/Application Support/Company/Product to ~/Music/Company/Product. If you add this to your project compile flags you just need to make sure that the installer copies the file to the music folder instead of the default one.
Still won't solve your problem of installing to the global and user domain at the same time :)
