there is no problem.. it doesn't work as a "line of sight" it works like your cell phone or radio, just on air waves. it's just a distance thing.
To clarify, wireless keyboards and mice work on RF (the same thing a remote control works with). There isn't an issue with mine on a pull out tray. Then again, my tray has a cord holder and I just placed my receiver right in front of my keyboard and out of sight and off my desk.