The only rooms that Disney could use are those that are either not declared - such as Riviera, or those that are part of the 1-2% of rooms that they keep control over. There is simply no way, contractually, that they can cancel rooms already reserved by DVC members, to sell them for cash reservations to the NBA.
Disney has declared only 97 of the 341 DVC rooms at Riviera. They would be allowed to use not only those 244 rooms, but also any that equal the number of outstanding points yet to be sold. The current declared inventory is 1,930,114 points, and DVC has sold 1,402,612 points as of April. That leaves 527,502 points unsold that Disney can use for cash rooms, but only DVC will know exactly how many rooms that is.