Thorn Raven. Not too fussed about the slacker head angle – I like it that way. Seat angle is fairly steep anyway – 73deg, so losing a bit won't eb the end of the world.
Forks are about 500mm axle to crown, so I deduce 6% (30mm) increase from 100mm forks.
I assume head tube and welds are sized purely in bending, ie if the wheel hits something at speed. Obviously forks compressing will take some, but the rest of the force is taken as bending moment across the length of the head tube.
So, a 6% increase in Bending moment therefore equals a 6% increase in stress. (A touch more if there's any shear, but I suspect its very little). And if that 6% knocks it above yield stress of 853 tubing, then it was under-designed in the first place. I'm only 80kg, so unless there's a 85kg weight limit, I should be right.
I'm going to fit a Crank Bros headset to knock a few mm out of the height, and as you say, if I run the forks with, say 40mm of sag, the overall shouldn't be much difference to 100mm.