A simpler solution would be just to have maybe a 10 level cap on the bosses. If someone is over 10 levels, then he cant attack the boss.
Really, there is no justification for 50's to solo low level bosses.
I like this idea best. World bosses are there to promote competition between raiders. To instance them would be taking that away. But the guy above me also has a good point. So maybe make it so after 10 levels you get absolutely no benefits other then a codex entry. That will deter most 50s from soloing them more then once.