John K Jordan
I've made knot hole repairs, not in a bowl but in flat wood, by cleaning up the hole a bit then gluing in a replacement knot shaped to fit the hole.
I've also filled voids in woodturnings by filling with pieces of wood. One was a long void extending from a bark inclusion so I cut and glued in strips of bark. I have never liked filling voids with some homogeneous substance but the strips of bark had enough variation to look natural. Depends on what kind of look you like.