I'd really like to see this, and I think it'd be pretty simple for someone who knows how the GUI elements work...
Basically, whenever I mouse over a heal, I'd like to see how much it would/could heal everyone in my party (or each of my desired targets in a raid, I guess...I've never been in a raid, so I can't really comment on that yet...a few more weeks and I'll be 60 and then...oh yes...)
What I envision is this:
Mouse over any heal, anywhere, any time. The HP bars of people suddenly show PURPLE (customizable color, of course) hit points above the normal green bar, showing just how much the heal is likely to help. Perhaps it's PURPLE up to the minimum, then lighter purple up to the maximum, so you can see the range. That way, when I do a pickup group with a Paladin and a Warrior, and both are getting hit, I don't need to ask "hey, how many HP you guys have?" then calculate for each of them at what point I should heal them.
It would also let me do a quick mouse-over of a few different heals, ranks, etc. to pick the proper one to not waste mana if I decide to heal before my best heal fits...
All you need is a mouseover tracker, a few HP bars with lower strata than the health bars (check out HoTBar, which I'd love to have work with this mod as well, but if not, oh well :)
Anyone interested in taking up the job? It'd be awesome.