if anyone really wanted a Law reference I would use obstruction in open play.
OK - let's try that.
[LAWS]law 11 Definitions: As the match progresses players may find themselves in an offside position. Such players are then liable to be
penalised until they become onside again.
In general play a player is offside if the player is in front of a team-mate who is carrying the ball, or in front of a team-mate who last played the ball.
Law 11.1: (a) A player who is in an offside position is liable to sanction only if the player does one of three things:
• Interferes with play or,
• Moves forward, towards the ball or
• Fails to comply with the 10-Metre Law (Law 11.4).
A player who is in an offside position is not automatically penalised. [/LAWS]
Prior to the tap, the wall is onside. As the ball disappears behind the wall, they become offside. The oppo, meanwhile, is still 8m away. Can we really call that the wall is interfering with play? Because it certainly isn't subject to the 10m law or moving forward towards the ball. If we ping this, then of course we need to ping the miss-pass that travels behind the dummy runner. We must be consistent - this is, after all, how the miss-pass was reffed in the 1970's, though the norms appear to have changed now.
[LAWS](b) Offside and interfering with play. A player who is offside must not take part in the game. This means the player must not play the ball or obstruct an opponent.[/LAWS]
Obstruction requires not only that an opponent is there to be obstructed, but also (contrary to davidgh's earlier post) that he actually is obstructed. If merely being ahead of the ball is problematic, all forwards are to be pinged as soon as scrums, rucks or mauls end with a backs move. We must be consistent here.
So this is not capable of being reffed in isolation; adopt this line and you need to sacrifice either perfectly normal and universally-accepted tactics as illegitimate, or else sacrifice your reputation for consistent refereeing.