Actually, KingMike, JCE and I were bouncing some ideas around a bit to that effect.
My FFIV User Options patch already makes use of a percentage that represents the ATB timer and replaces the Max HP display, so there's some proof of concept.
I was actually thinking about replacing that percentage with a meter, which could be toggled between ATB and Max HP.
If the meter was four characters wide, it could make for a pretty accurate percentage representation (each pixel of width represents 3%), and would allow enough room for five-digit Current HP display.
It's all quite doable, but I have quite a bit on my plate at the moment.
This is something I will be tackling in good time, though.