Encounter target window?
 

Im not sure what data is sent to the client, or if this would even be possible without putting some kind of dll hook into the client.. but it would definetly be useful to have a HUD window displaying all the mob names/hp in an engaged (or even non-engaged) encounter. Seems like it shouldnt be too hard because all that information is readily available by tabbing through the encounter.. somehow I doubt the client sends a request to the server everytime you press tab, so that data should be readily available.
Any idea if this is possible?

Not possible without hacking the client at this time.

And there is a VERY large step, both in ease and in acceptability/legality, between simple alterations to text and image files, and hacking data streams and altering DLL/EXE files.