I was just reading an article in ANN about what did the first editor had to say about Dragon Ball Evolution.
Basically back then, it was common for licensors to try to sell maga rights to movie makers as a way of earning “free money”. They had the rights but for whatever reason the product was never made (maybe too complex to adapt, not of interest for American audiences at the moment etc.)
What happen was that the rights expired and mangakas, earned money and that’s it.
What the editor says next, becomes interesting as Naomi herself has addressed the issue. He said that after seeing the disaster that was Evolution he consulted with lawyers and asked what he had to do in order to have veto power, (as basically if you sell the rights you have 0 say in what happens to the project after.)
The lawyers said that in order to have any say in the movie, the editorial would have to pay $45 million dollars (which seemed a lot for the editorial at the time) in order to be big enough to own a part of the movie and distribution rights. Therefore being able to decide what happen or not with the script.
Funnily enough, after this the deals of manga rights stopped for a year, as manganas were scared of what Hollywood might do to their works.
This might be why Naoko will never sell the rights, as in order to have the movie she wants, she would have to pay for it.