[jacquesm]

A tree is 'mostly dead', the inside doesn't do much, the outside is where all the action is and grafts are usually done with very young material so there will be plenty of support by the time it gets larger and heavier. The proportion of 'living:dead' material is much more amendable to grafting too. And it tends to be done with budding branches rather than a whole tree, in fact I haven't been able to find any reference of someone cutting down a tree and putting it back together, especially one of this size, the structural challenge alone makes it something that I do not think will work, especially not in that location with the tree taking the brunt of the wind load through that gap.