The first idea I have, looking at this, would be for an undercover spellcaster who doesn't let on that he can do magic, and using subtle enchantments (a small bad luck curse here, twist someone's mind to make them more open to his ideas there...) works his way into a position of economic power as a major metal supplier with contracts to most of the organizations that turn metal into tools and weapons for the anti-magic military groups.

Thing is, with spellcasters under his command working at key points in his mines and factories, they slowly weave enchantments into the metal they ship- individually the enchantments are small, somewhat easy to do, really just little more than targeting marks painting all the metal that they use.

With the aid of a large technomagical focus to amplify his power, this would let him (once a certain threshold of market permeation has been reached and a solid majority of current military enterprises uses such metal) he could unleash a more potent enchantment on it- oh, say, heating it to five times melting point for all of three seconds, enough to turn the entire opposing force into molten slag.

Could end up turning the war around and pretty well winning it without the mages even knowing what he was up to.