I only tried what you say once, and it's possible, if you work quickly. At that time I was trying a layered soap, and I wasn't quick enough to achieve that.
What works better for me is adding salt directly into the mold. Personally, I have used only individual molds, because I fear a larger piece may harden too quickly to cut neatly. If you have a spaced mold, it will be much easier.
This is how I go about it: sprinkle salt more towards the outside of the mold, then mix with a spoon handle, from outside towards the center.
Something I wanted to share is a Sunflower salt bar (SF at 60%, then PO, CO, stearic acid).
Only 2 weeks of cure, but since I had some time to kill today, I decided to do a lather test. It's got less salt than usual, you see here the 2 versions: one with 20%, the other with 40% salt. As you can see, it's not a big difference (each bar got a 30 seconds sponge rub).
Yes, the camera decided it likes better B&W pictures.