Some of it does work, I've got current group member health percentage added to the end of my group windows for Draco'Kyn UI.
That's what I'm using for it, works fine, far as I can tell.
I have had one report of a bug - Scribble said he had issue's when grouped across zones. I haven't been able to duplicate it, but I haven't experimented with combat across zones. Taking falling damage in one zone, with a group member in the other updated both group windows in my original tests, I still need to test "regular" combat with it, though.
edit: One thing that could affect this.... the data filter. I've got it set to FBFF, if I remember correctly. And no, this isn't listed in the UI customization guide, I found it when scrubbing the code against one of the default UI files, not sure which. :\
edit 2: ok, grouped w/ some guildies today while I tradeskilled.... while thier health bars reflected damage taken in other zones, the health percentage stayed at 100%. I'll try and see if it updates while in the same zone, not sure if the wifey will let me have any playtime tonite or not, though.
SOO..... it does indeed seem to be broken for cross-zone updates, at least.