I know that in some states, intentional torts are not insurable, so they cannot be assigned to an insurance company. But what if the assignment is from a closely held corporation to its sole shareholders? Would that be a valid assignment?
Insurable and assignable are two different things. An insurance policy will normally not cover the tortfeasor (the party that commits the tort). The person who would want to assign their rights would normally be the one who was harmed, not the one committing the harm.
Whether a closely held corporation could assign its claim to a shareholder will depend on the circumstances and the type of tort. In most cases I wouldn't expect that there would be a reason for the corp to assign the claim, but instead would expect the corp to pursue the claim in it's own name.