What's actually different in terms of support for VST3 on Linux? Has that been investigated by someone?
Yep, we're currently investigating and it's a considerable amount of work. The Linux VST3 SDK requires that hosts expose an event loop to the plug-in so it can register file descriptor and timer event handlers and getting this to work nicely with JUCE's existing event loop basically requires a complete re-write of the Linux messaging code. On the client side, we then need to hook into the host's event loop which again is non-trivial.
It's a high priority for us, but unfortunately giving exact timelines isn't something that we can do right now.
I found myself repeating WTF when reading that message so only wish you luck!