IRB (as it was then) issues a law clarfication 2014 covering non engagement in a maul following a lineout. Can't find it on the WR site, but this is what we have on our soc. website.
[laws]IRB clarification for teams choosing not to engage at the lineout
• if the defenders in the line out choose to not engage the line out drive by leaving the line out as a group, PK to attacking team;
• if the defenders in the line out choose to not engage the line out drive by simply opening up a gap and creating space and not leaving the line out, the following process would be followed:
- attackers would need to keep the ball with the front player, if they were to drive down-field (therefore play on, general play - defenders could either engage to form a maul, or tackle the ball carrier only);
- if they had immediately passed it back to the player at the rear of the group, the referee would tell them to use it which they must do immediately...
- if they drove forward with the ball at the back (did not release the ball), the referee would award a scrum for accidental offside rather than PK for obstruction.[/laws]