At first, I thought you'd need to go to Photoshop to do it, but I came up with a way that might work.
In LR, convert your image to B&W. Then click on the button below the tone curve and turn it into a Point Curve. Make two points on the curve and drag one to the top, and one to the bottom so that there's a vertical line between them. The left/right position of that vertical line determines what will be black and what will be white in the image:
If the tip above doesn't work, you can always try checking out online tutorials on different adobe lightroom tips and tricks. Here's one that was pretty easy to find, though you'd have to see for yourself whether you could get great use out of it. http://lightroomtutorials.blogspot.com/