So the immune system doesn't calibrate (for want of a better euphemism) to recognize it's own cells until fairly well along in fetal development & the major components of the immune system (antibodies, white blood cells, etc) are all produced by the bone marrow.
Those are the known details that cause me to wonder if this (below) might not be a viable approach.
Introduce the patients cells to a pig fetus before it's immune system has been set & it will be born with an immune system that recognizes both it's own cells & the patients cells as it's own.
Kill the patients own marrow (as per a normal bone marrow transplant) & transplant the pigs bone marrow into him giving him an immune system that recognizes both his cells & the pigs as it's own.
If my understanding is correct you should then be able to transplant any other organs from the pig into the patient without any tissue rejection issues.
I am only interested in the science so (any legal & moral issues aside) are there any major factors I'm unaware of to suggest this approach is a non-starter?
See Xeno: The Promise of Transplanting Animal Organs into Humans By David K. C. Cooper M.D., Robert P. Lanza M.D. Published 23 March 2000 by Oxford University Press, specifically the section on "The Induction of Chimerism Before Birth" from page 117 onward in this link.
That talks about my suggested first step (though in reverse), creating a human with immunity to cell rejection of a donor animals cells, which isn't much use to anyone who's already been born.
So I flipped the idea.
A Thymus transplant might be needed as well.