It seems that the interface
getPlaybackRegionHeadAndTailTime() for the ARA extension is not accessible via the JUCE interface. So it doesn’t seem possible to change the head time and tail time of the playback regions which are always null (nor to notify the host of this change).
ARADocumentControllerSpecialisation::ARADocumentControllerImpl class should implement the
doGetPlaybackRegionHeadAndTailTime() method inherited from
DocumentControllerDelegate (and we should be able to modify these values via the
ARAPlaybackRegion class I guess).