Suggestion about head transplant
I think that immune rejection is not the only major problem in head transplantation. 

We are quite skilled at suturing vasculature, but it's not fully solved yet how can you make new and proper connections in the spine? 

Concerning your questions, if you just took these naive lymphocytes, they would probably get destroyed by the acceptor's immune system pretty soon. If this wasn't the case, the approach would already probably be in use for other transplantations too. 

What you possibly could do is a bone marrow transplantation, where you would replace the bone marrow of the acceptor with that one of a donor. However this would cause the opposite problem, the donor cells attacking the acceptors body. 

For now, lifetime immunosuppresion is the only working way to go. 

To get a better idea about how exactly the mechanism of rejection works and the prospective future therapies, see for example: http://emedicine.medscape.com/article/432209-overview#a8