While phanx is right that it cannot be done easily, or even exact at all, it is, with a lot of work possible to get a general idea.
This is just a rough outline of what should be possible:
You will need a couple of players in the area of your target. This could work in a battleground/raid, but probably not if you are solo.
Every player can get its position and an estimate of the distance to the target.
With this information you could potentially get a rough estimate of the position of the target.
Your position and facing is easy to get, hence you can get the direction to the target.
You would need everyone in your group to be running the addon and targeting the same target to get any (marginally) useful results. The only scenario where I could see this being remotely worth the effort would be for RBGs, but probably not even there; when everyone targets the same enemy player, it doesn't really matter exactly where that player is located, only whether or not you can attack them. Also, in PvP nobody is just standing in one place, so by the time you get an estimate of someone's exact position, they've already moved.