sub EVENT_HP for players?
 

Is there something that exists like this for players?

I have looked (probably in the wrong places) but can't find anything.

What I am trying to do is to create a skill that fires off a buff when players get below a certain hp threshold.

In testing, I got it to work pretty well using a timer, but that seems "Hacky"

See spell effect 442 and 443

You should be able to accomplish your goal using them in junction with a spell script if needed for complex stuff.