The reason I ask is because I suspect that's more doable - it's similar to the double-teaming logic that already exists.
However, I'm not in favor of going any deeper than being able to set one defender to cover one opponent - as llamanunts pointed out, you're going to open up a huge can of worms. And he's also right that this may tilt things more towards the defense, and offenses are going to complain that they don't have a response ("If my guy is defended by their stud defender, don't shoot as much").
Overall, I'm neutral on the idea because I'm not sure it buys you much more than just switching around your starting lineup to create matchups, as you can do now.