This sounds very bottish. Having an addon respond to ingame events to have your character perform some other action could lead to a ban. Maybe others have other thoughts/ideas, but I would be careful about doing this.

Edit: As a simple workaround, I believe you could setup a castsequence on each character. (Lets assume you are a 3 boxer). You have to be careful not to spam the button, or you will cast drums twice.

Toon1: /castsequence Drums of Battle,,

Toon2: /castsequence ,Drums of Battle,

Toon3: /castsequence ,,Drums of Battle