This does not work all of the time and the details are a bit vague so give this a shot and let us know if you get it working. Set up WoW to play in window mode. Should your priest happen to die and your Spirit of Redemption pops up, minimize your window and wait about 10 seconds. Return back to the game. The Spirit will stay up, you will continue to heal without using mana and will not die.
don´t know if this works yet