I wouldn't allow it in that case. I wouldn't say I couldn't see a scenario where it could be utilized on a weapon in use, but it would likely be some sort of large turret or such where they disable it by cutting a hydraulic line or something. Not personal weapons in someone's hands being used though.
I can see cases where it could be used on weapons that are currently in use. The mechanic whispers over his group's commlinks, "Hey guys I noticed that the light repeating blaster that one stormtrooper is using has a cracked thermal discharge coupler. Draw his fire and I'm sure that thing will overheat when he tries to use it".
I think that kind of a use begins to usurp the Sunder effect and weapon damaging options for multiple Advantages and Triumphs.
There are often more than one way of accomplishing similar effects. I don't think that it is out of line to allow a once per game session talent to mimic Sunder or the same effects of a single roll of Advantages or Triumph. I kind of think that is the point of the talent. Instead of relying on luck and hoping for those Advantages or Triumphs the mechanic can try to purposefully make something fail but only once a game session. In fact that seems to me to be one of the weaker uses of a one shot per session talent that players could come up with. I mean sure you CAN make that blaster rifle fail but why would you want to blow your use on that?