The intention is that Stunned characters cannot enter Brawling combat and for simplicity, they are not currently penalized either if engaged.
Optionally, you may try out this rule:
A Stunned character that enters a Brawl for any reason (whether initiating or defending) will fight as normal but must roll 2D6, picking the lower score for their Brawling score.
The Stunned character will suffer an additional hit for EACH of their dice that score a 1, meaning they can take up to three hits (ouch!).
There's a chance the Stunned character is actually faking it to lure an attacker.
If either of the Stunned characters dice is a 6, they will inflict a hit on the opponent, even if they ultimately lose the Brawl. This could mean both characters are struck.
A Stunned character rolls a 2 and a 4. Their final score is 2.
A Stunned character rolls a 1 and a 4. Their final score is 1 and they take an additional hit.
A Stunned character rolls a 2 and a 6. Their final score is a 2 but they will inflict a hit on the opponent even if they lose.
A Stunned character rolls a 1 and a 6. Both combatants take a hit (and the Stunned character likely takes another hit for losing)