There's some discussion about that. Like in past GTA games, the health is most likely a part of the mission scripting (main.scm for old games, not sure about GTA IV). It was never really a part of one of those .dat and .ide files. Mission scripting seems a little out of the scope at the moment, but I don't doubt that it'll come up soon. The memory addresses are already being documented, and those are the things that trainers can modify.