A solution to NOT drill your track forks and have a normal front brake nonetheless

"*if there's a bit of a stem fitting into the steerer snugly, then there is no other movement possible for the whole thing but up/down the steerer tube"

Yes, but the idea that force is 'redirected' is wrong, putting a piece of 'stem fitting' into the steerer will not 'redirect' the force 'into a more or less vertical pull'.

Come on, [I]how on earth* would the starnut experience a relevant amount of shear force in this setup?

I don't think anyone has said it will.