I'm going on memory here, but, you have to attempt to dispose of the ball lawfully if you have had prior opportunity, but you do not have to attempt to dispose of the ball lawfully if you do not have prior opportunity.
As for throwing the ball, it is apparently still an unlawful disposal, prior opportunity or not - actually doesn't come under the holding the ball rule, comes under legal disposal.
So, if you have not had prior opportunity, you can, under the holding the ball law, dispose of the ball by dropping it, you can also hold it in. However, if you attempt to throw the ball you will contravene the legal disposal rule whether you are being tackled or not.
The less said about enforcement of the legal disposal rule, particularly relating to throwing being an illegal disposal, the better!
DS
I should add that it is a damned fine line, as TBR says if you drop the ball but are making an effort to kick the ball (ie: making an effort to legally dispose of the ball) it is not holding the ball. So, any player tackled who thinks they may have had prior opportunity is going to attempt to legally dispose of the ball - note they only have to attempt this, they do not have to succeed in their attempt.