Sean Houtman schreef
Look up some books by Alex Shigo, in particular "A New Tree Biology..."
where you can learn a whole lot about tree health.

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Not sure if this applies here. I am glad enough to have worked through "A
New Tree Biology" (essentially a collection of black and white pictures with
long captions/short texts), but it is hard work, what with Shigo's tendency
to make sweeping statements and his sloppyness in doing so.

Actually Shigo feels that when it is time for a tree to die it should be
allowed to do so. What does come forward from Shigo's work is that wounds
into sound wood should be avoided as much as possible and kept as small as
possible.

In general it should be a good idea to remove as much rotting wood from a
tree as possible, but not at the cost of cutting into sound wood. So the
general advice is to get all the loose stuff out of the tree.

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Any thoughts from apple experts?

Sloan(e) has some illustrations in one of his books about wood or trees or
something, I have forgotten his first name, (Eric?) as well as the titles of
his books. What you describe is an established method of propagation of
apples.

Sean

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Perhaps:
Eric Sloane
"A reverence for wood"
This has a bit of stuff on apples in american folklore

PvR