With, or without, the coating? I would guess that they might be a little scratchy with the "shell" on (and they have a slight point at one end) ... a single scrap along very fine sandpaper should sort that out.
Without the shell they won't last a very long time because of the high oil content, and may (I haven't tried it, so I don't know with these particular seeds) discolour slightly where they touch the soap (most botanicals do - they tend to turn brown, which shouldn't matter too much with these seeds).
In other words, as a temporary decoration on top of soap, I personally wouldn't, but I can't see the harm in it.