If the size is only 0,5mm on a 16x20" print I'd suggest knifing the print. Anything you do on the neg will end up a lot bigger on the print than a knifed speck.
I have not done this, but I have heard that a pin or knife-point prick on the film will optically diffract light around that area, leaving a white spot on your print for retouching. I have used a fine point red marker for LF film with some success. As someone mentioned, doing this on smaller formats means bigger areas to retouch when enlarged.
Tom, on Point Pelee, Canada
Ansel Adams had the Zone System... I'm working on the points system. First I points it here, and then I points it there...