I've never ridden a penny-farthing, but one technique I've seen used for going downhill is to put both legs over the handlebars so that the rider's legs are sticking out front. That way your feet are off the pedals to facilitate coasting, and if you fall you're likely to land on your feet instead of on your head. Try to get your weight as far back as you can, as that will make flipping over the handlebars less likely.
The penny-farthings I've seen had a spoon brake. A lever that pushed a paddle against the tire. You could probably put a regular rim-brake of some sort on there, instead.
Be careful with braking on a penny-farthing, especially going downhill. It's easy to go over the handlebars. Gently apply the brakes to control your speed, don't yank them and try to slow down quickly.
Or you could walk it down the hill... Downhill penny-farthing riding is a good way to break your neck or skull.