I have to find a rules quote (away from resources) but from memory: Yes, though he'll appear there dead. With the way you worded it the event that triggers has to happen first, and going from full to dead is instant. Then it will recognize that you are indeed at "below 20 hp" or whatever you set it to, and warp you.
A better wording would be "warp me if I would take an attack that would kill me". That puts the contingency on if you'd die or not.