The idea that a longer fork will overstress the headtube and the frame will break is nonsense. A 20mm increase on a 500mm AC length is 4%. That’s utterly insignificant.
It’s even less than that, really, as the moment arm includes the wheel radius (ie distance from bottom headset bearing to tyre contact patch). So on a 29er, add another 14.5″ (370mm):
20/(500+370) = 2.3%
As the man says, utterly insignificant.
I suspect that manufacturers’ warnings against over-forking is more because it’s an indication that the rider may be using the bike inappropriately. I have some sympathy with them from a warranty standpoint, but even so you’d have to be *really* overdoing it to exceed the design safety factors.