As far as I can tell after taking a short look at the source code, there’s nothing in JUCE that supports it. I imagine there’s quite a bit of work to be done, involving changes to juce::AudioProcessor API. On top of that, for cross-platform preset compatibility there’s an issue of marrying the AU preset model to the plugin’s own model (that may have nested hierarchy as opposed to flat AUv3).
That being said, it would be great to hear something from the JUCE team about this issue…
// Also mentioned here: