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.
As you said yourself it'd deter most level 50's from soloing them, but would that also deter the people who do it for fun? Because that was why I soloed them once - to see what the bosses looked like.
I was wondering myself what to do with the "ops race", but I just couldn't find a better solution than to instance the bosses and discard the "ops race".
In my opinion this prevents most bad things from happening. And I'm looking for a system that makes it able for everyone to say "Hey! We'll do this world boss on monday!" - and then if a level 50 had just soloed the boss monday morning, it would be impossible for these guys to do it. This system with instances would allow casual players, who don't play that much, to pick a date in the calendar with their friends and do the boss - I think that things like these would open the game for a much wider audience.
But you know - it's just an idea