When I was laid off in a particular snotty fashion, I got a phone call a week later asking if I wouldn't mind coming in to brief the new IT guy. I declined.
The correct answer is that now I'm no longer employed by you, the consultancy rate is (5-10 x previous rate) and you would be happy to help, but given the circumstances, the terms are payment in advance.
Obviously if you are still in the exit process and arguing about the package, then the same can be done on severance, again paid in advance.
If they want it bad enough, they will pay. If not, then you tried to help, but it becomes someone else's problem. I think its called cause and effect.