Just built a 24"er up.
I'm running a Echo fork with a 5mm thick wall on the steerer tube.
I ran into the issue for routing the front brake cable through the fork to allow bar-spins and tail-whips etc..
Here is my solution.
~ Barrel adjuster from a roady brake caliper (the threads match the star nut).
~ Upper and lower outer brake cable sections.
The brake feels great (no slop) and the stem stays tight.