I would suggest an overlay. Bear in mind that a grey stripe is not a tragedy. If your silhouette is black and the screen is white, you can adjust the lumakey to key out grey. What you don't want is a black stripe, which could result if you don't butt the edges perfectly.

As long as the silhouette is substantially darker than anything else, you can get what you want.

Getting the room right will be just as important.

Edit. It's important to have only one light, a point source, as far away from the screen as possible. You want to get the light rays as parallel as possible to make the dancers' shadows have sharp edges. If you're using an open face unit, watch out for multiple shadows.
Make sure that no other light falls onto the screen. So, no other lights in the room and keep the screen away from light coloured walls and ceilings. If necessary put black cloth, or a matt black surface, on the floor to prevent reflections from the floor onto the screen.

Keep the dancers as close as possible to the screen, the further away they are, the softer their shadows become. Remember you're filming the screen, not the dancers. If they're wearing any clothing, make it dark. Light will be reflecting off the screen onto them and dark clothing helps prevents it from bouncing back onto the screen

Watch your aperture setting, the large area of white may confuse your auto-exposure into under exposing.