OK. It seems that GarageBand won’t load any AUs that are not “sandboxSafe”. This flag must be set in the Info.plist file:
I’ve now updated the Projucer with an option to do this for you:
By default “sandbox safe mode” will be disabled (to remain backward compatible with older plug-ins) - except for PiPs, where it will always be enabled. As the JUCE plug-in demos are all PiPs, they should now load in GarageBand out-of-the-box.
If you want to try this out, you will need to re-build the Projucer. If you are testing one of the example projects, you will need to re-generate the .jucer file from the PiP (File → Open Example → …). Also, on my machine, I needed to reboot after the plug-in was built before GarageBand picked up the change.
This change will be available on develop shortly with commit fefbbc8.