Neurotic
I plan on living forever. Or die trying.
Sure it would, you can define the trigger, just not something that depends on something you (as a player) cannot know. For example, you have a reaction that does 5hp. You cannot define 'when it would kill the opponent', but you could say when it look near death and talk to DMthat it means under 10%? 5%? whatever YOU think appropriate of total hpBut then you're not solving the delay of play posed by PBP. The entire point is to speed these kinds of things up...
You would get your reaction, just not guaranteed death and your reaction would be used regularly.