Unfortunately I don’t think this can work, but there’s also not a straightforward workaround, at least with the current design.
The problem is that the EditController instance doesn’t know anything about the particular AudioProcessor instance that it’s wrapping until connect is called, but the host may query the edit controller’s interfaces before calling connect. Given this, there’s no way to call through to the VST3ClientExtensions of the AudioProcessor to check for additional interfaces.
We’ve had at least one previous request to implement the IContextInfoProvider extension directly in JUCE. If a sufficient quantity of users would find it useful, we could investigate and find out whether this work is actually feasible. You could try opening a “built-in support for IContextInfoProvider” feature request to gauge support.