You've gotten some good advice. Another thing you can do is to bleach the print before it is selenium toned. Get some tincture of iodine and a toothpick. Sharpen the point of the toothpick if needed. Dap toothpick in iodine. Touch the point of the toothpick lightly to the spot for a second or so. Wait a little while and repeat as often as needed. You will have a redish yellow stain on the spot until you refix, which you have to do to make the bleaching permanent anyway. After re-fixing, wash, tone, wash, hypo clear, wash, dry... You then spot the resulting white spot with whatever you'd like.
Another thing to try would be to tape a sheet of drafting film to the negative base. Do your spotting on that, and then print the sandwich. That way if you screw up the spotting, you'll not have ruined the negative.
Someone mentioned india ink. I use this with some gloss medium for retouching prints. It works very well. Unlike Spotone or Marshall's dyes, however, india ink is waterproof, which mean that you can't wash away your mistakes. With prints this doesn't bother me, but I'd be more hesitant with negatives.