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The bottom end of a Unicrown steerer is usually plenty thick enough to be drilled, the sides are thick to withstand the welding and then the fork stresses, and the front and back end up much thicker than they really need to be because the tube has the same wall thickness all the way around. If the wall is 2mm or more, it should be fine. If it's 3mm, you can probably even cut a spot face for the head of a tube nut and use a normal Allen key brake.
I have an undrilled track fork on a Graham Weigh frame. It's like a unicrown but with flattened straight fork blades, so it's actually really nice.
How can I tell if it would be safe to use with a brake if I got it drilled? Would I need to check if the crown is reinforced somehow or should it be alright?
There's definitely room and clearance for a brake on the crown.
Thanks!