The head injury exemption in terms of front row replacements is also open to abuse. It was clear from replays that Oghre took a blow to the head and he required a lengthy spell of treatment and Wasps would have allowed him to continue if he could have done so safely, so no-one can seriously suggest otherwise. Wasps were even willing to risk a loose head at hooker to avoid uncontested scrums, and it was really West's less than convincing answer when asked by Barnes that averted this. Credit to Barnes for managing the situation correctly.
However, if a team really did want to manipulate a situation to their advantage as with France in the infamous 100 minute match (also Barnes), if a medic says it's a head injury then a ref, whatever his suspicions, is not qualified to overrule that view, meaning that the team stays at 15. I'm sure physios, doctors etc have been sent on to attend prop injuries before now with a brief from the DOR saying "If the ref asks, he's definitely had a knock to his head as well as whatever else you might need to treat him for".