I don't think I've ever seen a 24" fork with U or V mounts.

If you do mount a brake plate I'd go with a U plate on the back of the forks and run the cable up the steerer tube, it's probably the best compromise of performance and tidiness. The braking will force the plate against the forks. If you run a V brake plate in front you'll have the cable hanging out all over the shop and you'll need to secure the plate at 3 points.