It already is. Crits to the head very often cause Stunned or Unconscious conditions. An untrained brawler would make KO by accident (ie a Crit). Seems realistic to me.

Couldn't you simply give the Pummel quality to fists when trained in Brawling?

To answer the first question, I would say no, just because the Crit deflection rules are in the Armour section of the Consumer guide. The rule seems strictly tied to Armour, in intent.

Berzerk Charge also gives you +1SL on your Successful Melee Test when you charge. That means you will win the Opposed roll more often and deal 1 extra Damage due to SLs...