NEF and IView Thumbnails in Grayscale

Is there any way to have my NEFs preview their thumbnails in Grayscale (Black / White) in iView? They were originally shot in color and right now the thumbnails show a color thumbnail. But for reference, I would like them to show as Grayscale thumbnails in iView instead.

Is this possible in IView? Or do I have to alter the actual thumbnail embedded in the NEF? Thanks.