Stumbled upon this yesterday... I rarely shoot people, but Marcia and I are putting together a book of her poetry, and she wanted an image of herself for the back of the book. I took an image of her I had processed in Lightroom, created a Virtual Copy, and converted that to B&W. At first I didn't understand what I was seeing, but there was still some colour to the image, in her eyes and her lips. For the colour image I had accentuated these areas using the Brush tool, and I had set the colour box to a salmon colour for the lips, and added a hint of blue to her eyes. When I converted the image to B&W, these brush tool overlays stuck. I removed those two brushes and the image became truly B&W.
At first I thought it was a bug, but then I got thinking about old B&W images that have been hand-coloured. Photoshop would definitely be better for this, but if you had a steady hand and a pen tablet (and more talent than me), you could create a B&W image in LR and then hand colour it. This would be different than selective desaturating, because you can add colours that weren't there in the first place.