I finally had some spare time to write a JUCE VST3 Wrapper for Win 32/64 and Mac 32/64.
While writing the VST3 Wrapper I decided to extend the existing Wrappers for AAX, RTAS, AU, VST to support Sidechain Inputs and Aux Outputs, too.
Jules, if you're interested in integrating this into the official JUCE version, I can send you the source files and a detailed documentation.
Just let me know.
UPDATE July 13 2015:
1. Existing RTAS, AAX, AU, VST, VST3 Wrappers extended to enable multiple Input and Output Buses (sidechain In / Aux Out)
2. A new ChannelLayout class to handle information about connected channels and buses
3. Slightly modified versions of the existing AudioProcessor and Utility classes to handle the new Bus/Channel capabilities
4. Info about additional changes (not necessary for the multi-IO features): Some of the Wrappers contain additional fixes or improvements