woudln't be possible for mages mainly because of clearcasting
I don't think it would be possible for other classes, but dr damage actually shows how many casts till out of mana, although it is not updated toward the amount that you used
The same is true with holy priests talents that involve crit. The point is that it's still a useful stat to watch.
I.e. if TimeToDie>TimeToOOM(without freecast talents taken into account), you should be concerned.. if you get a few freecast heals, than that means timeToDie(decreased)>TimeToOOM(stayed the same). You feel a little bit better.
Perhaps a few more freecasts happen, and now TimeToDie<TimeToOOM.
I did a rough version of TimeToOom here. It uses the same algorithm as TimeToDie did, so the result may be a bit rough, but it's a starting point and can be fine tuned. As mentioned above, this'll never be quite as useful as you'd like it to be, but it's at least something.