I think people who suspect the shaman flesh as being the origin of grafting are probably right.

If the Lands Between's process of grafting is closer to the plant version than the fleshy one, the process of making a jar saint is using the sliced up rootstock (shaman) as a solid base for buds (other people in pieces).

But mostly it's because of the term "whip grafting" and the use of the tooth whip.

Unfortunately actual whip grafting doesn't seem to resemble the potted people process that closely but Miyazaki does like to add completely literal interpretations on occasion.

I was trying to find something about causing an infection before a (plant) graft, maybe something about the increased immune response helping the process but I haven't seen anything yet. Honestly from my basic knowledge of biology that sounds like a recipe for rejection but who knows, biology is weird.

I was thinking the use of the whip was like a type of flaying, removing the outer layer like you would remove the bark before a graft. Which, now that I've written that, it sounds exactly the same.

I feel like Miyazaki read up on tree care (husbandry?) and said "man wouldn't that be fucked up if it happened to people"