Could it be that the SilenceInProducesSilenceOut flag is not referenced at all in the VST wrappers? If so, is there a reason for this?
I don't think VST has an equivalent flag. IIRC it just uses the tail size to know whether it can optimise silent blocks.
Hm, there's the "noTail (bool)" method in the VST specs (which was removed from JUCE's VST wrapper some time ago), which, according to the docs, should do just that ?