Here are patches to add RTAS side-chain support to JUCE:
https://github.com/soundradix/JUCE/commit/489d066e7a83bb46fcc2d0cf1ad7be6c449c5dd3
https://github.com/soundradix/JUCE/commit/433c76bd1d1c11bc50cd0575257cb9112c923103
https://github.com/soundradix/JUCE/commit/8f86089cb3d967329c7d8bc5d6eb49c7fe76208c
Feel free to use / merge to JUCE / etc.
Cheers, Yair