Wouldn't this work? If your PctHPs are zero, you're dead and this will /exit.
Code:downshit0=/if (!${Me.PctHPs}) /exit
Is there any other time you're PctHPs=0 when you are not dead (like when you're zoning or logging in, for example)?
Actually maybe. I have some healing "downshits" that often try to cast following a zoning on me sometimes, but almost always on the pet.