There are probably lots of ways of doing this. The plugin is only one instance (probably) of your binary with multiple instances of the AudioProcessor and Editor classes.
So if you do something that’s backed onto a global variable then you’ll be able to share data.
Doing it directly is probably ugly and fraught with tripwires for the fast and unwary. However maybe something like a class GlobalConfiguration wrapped with a SharedResourcePointer<> or other singleton arrangement would work. Perhaps if your GlobalConfiguration object used a JUCE ListenerList so you can find out when one of your objects calls setGlobalScaleSetting(…).
There might be some existing JUCE class that’ll work. You maybe could look at PropertiesFile as well… I don’t know if it has the right notification behaviour.
Or you could use a common message bus arrangement between your objects or some kind…