Such a use of Discipline requires a Stunt.
Evocation already, in effect, adds a 'trapping' to the Discipline skill to allow it to control magical energy. Why would you need another stunt to do so? Note also that the example I referenced did not indicate that Guns could only be used that way if a trapping was added via a stunt...
Again, this is just my opinion, your GM might disagree, and that's fine. But given that DFRPG encourages coming up with creative ways to use skills, I think that this as a house rule would be easily within the spirit of the rules, and would be reasonable from a game balance perspective. After all, if you can justify defending against lockpicking by using Guns skill, how hard is it to justify defending against spells with magical skill?