A scheme used on many BMX bikes is a "detangler" disk on the fork. It's actually two disks, surrounding the fork shaft, arranged with flanges such that pulling up on the upper disk lifts the lower one.
Both disks are attached to cables, with the upper generally attached to two cables which lead, via a Y adapter, to the brake lever. Two cables from the lower disk lead to the two brake cantilevers.
These are fairly common on BMX bikes just above the cheapest ones, so you could likely view a version on bikes at a department store, and certainly any bike shop carrying BMX bikes would have some.
These are available online from several vendors as replacement parts, or you could salvage a unit from a scrapped bike.
(And I'll note that another option is to use a scheme similar to that used for the front brakes of "detangled" BMX bikes -- run the cable through the (hollow) center of the fork shaft. For your case you'd still need a small rotating cable connector at the top of the mast, but these are fairly readily available as machine components.)