DGM,
A friend of mine had a 12 bore island lock Purdey ejector which suffered damage to the head of the stock & wanting to save the nice wood in the butt & preserve the originality as much as possible had exactly what you describe done.
The result was a virtually undetectable repair but the gunsmith who did the work said the job was many times more difficult to do than a standard butt to sound head transplant & he never wanted to do another.
I'm sure the fact that it was done on an island lock gun increased the level of difficulty somewhat but transplanting a new head to the original butt makes it extremely difficult to maintain drop & cast dimensions & 0 room for alignment error on any type gun.
