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A Chicago screw might work. If the shank is a bit narrow you could araldite it in or tape it up to make an interference fit. You could even cut down a rawlplug so that the compression of the screw expands the plug into the hole to get a good grip.
EDIT: just noticed that the front-right item in @Constable_Savage's image looks like a Chicago screw, although I'm not sure if that's a screw or press fit.
I will never fit a brake to this bike. Never. Good suggestion though. Not too thrilled about removing more material from the forks but will consider it as it's the only way it'll sit flush.
Right now I'm thinking of something like a carriage bolt, see image below.
I'll have to grind away the square part & then screw it on to brake bolt nut holding the bolt from underneath the steerer tube with some needle nose vice grips.
Maybe that wouldn't look to bad?