In a U19 match on the weekend I observed the number 8 unbinding with a single arm from the scrum once the ball was at his feet so as to obstruct the opposing scrumhalf and shield his own scrumhalf from being tackled.
Law 20.3.g - indicates that if a flanker widens their stance to obstruct an opposing scrumhalf the appropriate sanction is a penalty kick.
Does this also apply to the number 8?
No, there is no such law to prevent the No 8 shielding the ball in this way. He has to keep a full bind, but one arm is sufficient. The bind must be continuous from armpit to wrist around the body of one of the 2nd row.
Remember also that the offside line for the defending scrum half is the ball and both feet must be behind it at all times
The Rugby Ref