I had some Nasturtiums seeds and they are pretty hard and tough. What I did was took the seed in my hand and gently rubbed it on the concrete to file away at it. But be very careful you don't go too deep and expose the soft seed in the inside. About 2 seeds I did it too much and the little soft seed was exposed but they still did sprout.
Also I have seen people just do a little nic into the seed like you were saying.