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@gbj_tester
Two immediate problems spring to mind; the thread roots are a stress riser, which is a bigger problem in aluminium with its short fatigue life, and the fine threads might shear, or at least wear fast, when the large diameter headset locknut is tightened against the upper cup. The combination of large diameter with small thread pitch gives a very small thread helix angle, which means a high thread shearing load for a given tightening torque.
tl;dr
I wouldn't. I think forks from the olden days had steel steerers for a reason.