As an alternative, I once raised this issue and referred to times when Harry would bring up a shield spell before a fight, but still have enough stress left to maintain the spell during the fight.
The advice I got, straight from Fred, was to treat the start of a given conflict as the start of a new scene, so that the wizard's stress track cleared once the fighting started.
Expanding on this, and using the often-overlooked rules that, once conflict begins, you set out what teams everyone works on (ie: a group of 3 PCs against 4 NPCs, 2 PCs take on 1 NPC each, while the third takes on 2 himself), you can make the case that once a particular pair of sides have finished fighting, that's the end of one whole conflict, and when the characters start fighting someone else, that's a separate conflict beginning.
For example, say in the course of a climactic battle scene, your PCs are fighting a large force of Red Court Vampires, trying to stop them performing a powerful ritual.
The PCs arrive at the scene and fight their way past the outer guards. That's one conflict, leaving some PCs with some stress boxes filled, but no serious injuries.
Next, they come to a group of elite guards led by an important Red Court Noble. That's a new conflict, and the PCs' stress track clears. At the end of this, they've each taken a mild consequence and some stress.
After that, they finally reach the core ritualists and dive into battle to stop them. This is another new conflict, so their stress tracks clear and only the mild consequences remain.
Using this method, you can set up large-scale conflicts without having to worry about your PCs, wizards or not, using up all their stress and being unable to take part.