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Others I am sure are going to howl it down, but to me it isn't out of the question to have a second official for that purpose.
They would be in charge of the set piece (Scrum & Lineout) while the main official waits on the defensive offside line to police that.
Once the set piece is over the set piece dude drops back behind the attacking backline until the next stoppage.
In the pro era, with so much more than just national pride at stake (ie $$$ ) one Ref is just never going to be able to monitor what is going on with pro super fit and fast athletes playing on the same sized area.
Food for thought anyway...