I've printed over sharpie and it works fine. That's on regular printable dvds, not Watershields. If you think you might want to do this, then use an "ultra fine point" sharpie and print small and inconspicously, say around the outer edge of the dvd.
Another interesting technique I discovered is that if I print the wrong picture on the dvd (because I didn't have anything written on it and guessed wrong), I can wash the ink off (just run it under the tap), let it dry and then print the correct artwork over the old picture.
The old picture is still there, but its very faded, and the second printing will usually cover it good enough to win out (depending on the colours of both).
While I'm at it I might as well mention one more printing trick I found out along the way for regular printables which you are making for little kids to handle (eg backups of the real dvd). If you don't want to go to all the trouble of spraying fixative on them to make them waterproof, then just wash the excess ink off and you are left with almost waterproof faded artwork on the dvd. It's a waste of ink of course, but it sure beats the spraying process or cleaning up the mess a toddler can make with an unsprayed dvd. The pastel artwork works just fine for little kids who can't read yet -- they can see easily which dvd they want to play(now that 2 year olds have their own dvd collections and even their own dvd players).