okay, it's canon that hexing is mostly post WWII stuff.
perhaps that's because during WWII, wizards were constantly taking down tech, and so anything from that era has been 'hardened' to resist hexing? Whereas more modern stuff hasn't been.
Consider that if you were in Europe during the war, the temptation to take down an enemy bomber - and to know that even if you got into trouble, you could claim that it was an accident - would have been constant. And not just of the enemy; wizards would have had the contacts to work out that bombing cities was counterproductive, so blowing the engine on one of your own sides bombers is saving the lives of the crew and the lives of the enemy civilians.
That also fits with what we know of tech during the war (remembering one early raid where of 19 bombers, 17 had to turn back with engine trouble and the remaining 2 got lost, found a target by accident that they then missed).
Uboats, mines, tanks, mtbs: all would be constantly being fried. And so hardened.
So how about a technomancer whose job during the war had been to improve the machines so they couldn't be hexed? Lots of iron, running liquid (why shouldn't the coolant systems in engines also count as running water for spell purposes?), thresholds and so forth.