Ironically what you're describing is very much like the traditional WHM ability 'Sacrifice' (or Martyr as it was called in FFXI) from past FF games, where the caster sacrifices some of their own HP to heal another party member by double or triple the amount. As far as I understand it, THM's original form at the start of FFXIV version 1.0 did indeed have a spell similar to that, but it was removed when the classes and battle system were redesigned in preparation for the addition of Jobs.

Also the CNJ class quests deal with a npc who actually uses this very ability, which slowly is killing her, but the attitude given in those quests was it was actually something taboo that should not be encouraged or taught, so I think this kind of ability probably won't be seen as an actual class in the game I'm afraid.