Well, if everyone used the Ward system it would be easy to write a plugin to do so, but with everyone implementing their own, it is nearly impossible to do so.

Even thoose aren't using the ward engine..

EDIT:
But where to display the name? hudhint? seems legit! but I know a better way(to avoid hudhint spamming lol)..
some of you will probably know (if debug mode is active)that entities will display their classname directly at their origin
we could take use of it. there are 2 ways.
1. create an entity with preds awesome CEntity framework and set classname to players name
2. try using a existing entity like trigger_multiple and change classname
then try sigscanning CBaseEntity::EntityText() or other stuff (I'm not completly sure if it would works - but it's worth a try)
by using one of thoose methods we also won't need to loop trough every player and check distance and other stuff, just hooking CBaseEntity::Touch() should do the trick

wut?

I've sigscanned CBaseEntity::EntityText and NDebugOverlay::EntityTextAtPosition but both didn't worked :/ ..seems like there is some debugging mode only check inside them. If we know the function we could detour it