Massachusetts General Hospital is launching Boston’s second hand transplant program, hoping eventually to pioneer a new way of replacing patients’ limbs without subjecting them to a lifetime of dangerous antirejection drugs - an advance that could turn a rare operation into a routine one. The idea is to transplant a donor’s bone marrow along with the hand to trick the recipient’s immune system into accepting the donated hand as its own. The hospital has had early success with this experimental approach with certain kidney transplant patients.