There are relatively common use cases for downsampling audio that is already oversampled. This may come up in some synths for example. It would be nice to be able to use juce::dsp::Oversampling for this, but it provides no API for supplying already oversampled audio data.
Actually a found a reasonable work around. Calling processSamplesUp() once after initProcessing() and then retaining the audio block reference seems to work fine here.