Well Tibetan monks are suppost to be able to leave their body via mediation. Maybe Locke learns to do the same thing?
I'm not saying it's not possible - I just think that's a big wrench to throw in the mix on top of everything else and pretty difficult to cover in the time they have left. If he had started out as a tibetan monk, then maybe you'd have something there.
My theory is that Locke isn't dead when we see him in the casket. Nobody came to see the body except Jack and Ben, so I think Ben set this up to jerk Jack out of his funk. Think about the darts that Sayid took - he looked just about dead, but was really just heavily sedated, so maybe Ben's been hit with the same stuff. It's not like Jack took his pulse. I wouldn't be at all surprised to see the Oceanic 6 hop on a boat along with Desmond and find Ben alive and well on board.
Just some additional food for thought - Locke is posing as Jeremy Bentham
. The real Bentham's dead body was put on display with a wax head, so could be something similar going on here. Maybe Locke never even leaves the island - Ben just pulls together a wax replica in a coffin to get his plan rolling.