Roel,
you certainly have a very clever designed drawer (knee-wise). Congratulations. I can't do the same though because the height of the drawer is actually less than my foot-knee distance. Off to find some bricks!

Scott,
unfortunately the drawer mechanism doesn't lock, but I don't think it will pose a problem. As for the weight, you have to calculate the weight for the drawer as well as the keyboard itself and the playing force. I found a pair of Hettich (Made in Germany) rails that supposedly can hold 45kg. The drawer itself is maybe 3-5 kilos, the Casio is a toy, I think I have enough clearance. Only problem is that the keyboard is bouncing up and down a little, because the drawer is flexing. If it continues like that and becomes a problem, I'll have to find another drawer material that doesn't flex that much. Drawer costed only about 8€ so no problem.