That's a really, really interesting idea and I like it a lot. You would also allow teams to draft the rights to players who did not declare, of course. It certainly would widen the field.
I wonder how high school prospects would feel going to college knowing an NBA team already had their rights. They might be disinclined to play for their college teams... I'm all for it, of course, because I think the NCAA is a criminal organization and that 18 year olds should be allowed to make their money.
I wonder how young speculating teams would go.