I wanted to make a function that would calculate the rate of hp loss, and turn it into a "seconds left to live" If that is < 5 then display health in big red numbers.
Doesn't look like it's gonna happen
Also, I already saw the DogTag spec page. The math functions were absent.
Not possible, but you could design an addon to do it. I'm not sure if DogTag lets you register new tags handled by your addon, but I suppose you could hook DogTag or the addon using it to try and implement the system.