There is a plugin that would add a cool effect to pvp it adds a health bar above the players head. It may be outdated but I think it is cool!
I suggested something in mcmmo that, unbeknownst to me, allowed you to see the players health. I don't think we're for health bars.
I think i found the plugin you are talking about and it is inactive. last update is august 26, 2011. It is also a spout plugin and would probably show health on top of player's characters, not in chat via command.
Watch as the wild imaxorz frolics in its normal habitat. What's this! He seems to be stalking a harmless strangecow idea. The imaxorz pounces in the idea and rips it apart! Haha just kidding your right and is outdated
PLease don't add this. It would slowly but surely ruin PvP. A big part of PvP is tricking your opponent into thinking you're winning, and if they know your health that aspect of it would vanish. Trust me, all I do is PvP on shadecrest, it won't turn out good.