Can a plugin (so a subclass of AudioProcessor) can access the current AudioDeviceManager of its host application ? (assuming the question makes sense…)
No, because the host application doesn’t likely even use JUCE. (And even if it did, the standard plugin APIs have no facilities to get access to something like that.) Why are you interested in being able to access it?
Beauce I merge code that run in a subclass of AudioProcessor, with code that actually also derives from AudioAppComponent (a bit messy, but this is the way right now…) I saw the AudioAppComponent( AudioDeviceManager& add) constructor, assuming it could be used to access a AudioDeviceManager like representation of the current audio context.