It isn't a recommended practice, but people have done it. It will probably reduce the range, possibly drastically. In the worst case, it wouldn't work at all. Accuracy can be affected as well, although this can be minimized by calibrating the sensors with a mirror after installation. It is best if the Leap Motion device is right up against the glass to reduce reflection of the IR illumination from the underside of the glass.
http://blog.leapmotion.com/how-to-build-your-own-leap-motion-art-installation/