Alright, so I already have a simple House Rule for any of you wishing to try it out when playing Valtyr Chronicles. This is completely up to your GM and entirely optional.
Any time someone wishes to use the Voss to cast a spell and they do not have their focus (usually the silver bracers given by the Citadel), they may do so but at a heavy cost to their life. Fictionally, the Voss is so volatile that sorcerers must channel it through their focus when shaping their magic. This allows them to safely use the Voss to power their spells. However, when a sorcerer is bereft of their focus for any reason, casting even the simplest magic can cause excruciating wounds. These wounds manifest as burns or sores that cannot be healed by alchemy or Devotion's Healing Touch power; they must be healed naturally, making the process take that much longer.
In game terms, if a sorcerer casts a spell without his or her focus, they lose 10 HP for every VP it would have cost to cast the spell. So, for example, if a sorcerer wanted to cast the Pacify spell, he would lose 20 HP instantly (since the spell costs 2 VP to cast normally). Casting silently without a focus is impossible unless the character makes a Spirit skill check (SR 4 or greater). If the character would shift to Dying as a result of the HP loss, they can still perform the action, but doing so immediately forces them to mark 1 box next to Dying.
Not that I recommend your players try such casting, but in a pinch, it's good to know that they can, albeit painfully.
Good gaming all.