I think that Lebron would be a little effective on Rondo, but it would tire him out sooner. Rondo is a load, and he is better at penetrating and dishing. As great a defender as Lebron is, this would be too much energy for him to use on just Rondo.
CLE needs Lebron's Offensive Punch more than anything else, I think.
Interesting concept, though. Roy Hobbs pointed out a similar situation a week or so ago in a different thread if we meet LA, with Kobe guarding Rondo in such a series.