Well, I've just ordered my first high wheel bike, and have not ridden one yet, but in preparation I've watched hundreds of videos and read every description of riding techniques I can find.
There appear to be five techniques for slowing the bike on steeper grades. Two involve brakes: either a spoon brake fitted atop the front wheel, or caliper or disk brakes retrofitted to the bike. These both seem inherently dangerous, as the rider's center of gravity is nearly perpendicular to the front axle, and abruptly slowing the front wheel makes it likely that the rider will be propelled over the handle bars--the dreaded header.
A third involves dismounting the saddle, and standing on the mounting step on the back of the frame and using the other foot to rub against the back of the front wheel, which seems awkward to me, but again, I've no riding experience on which to judge that.
A fourth technique is to dismount the saddle, stand on the mounting step with one foot, and drag the other foot on the rear wheel, which I've seen done in several videos with good effect, and that seems a less awkward position, at least to the casual observer.
The fifth, and most common approach, is to exert back pressure on the pedals to prevent the bike from gaining speed in the first place. Obviously, if the bike has already passed a safe speed, this becomes less and less effective as the pedals will eventually spin too fast to keep one's feet on them at all.