Originally, I thought I would put an encoder or something on the steering column and keep the pump off unless the wheel is being turned, but after a quick brainstorm with a colleague, a simpler approach is to turn the pump off whenever I push on the break pedal. That way, when I'm breaking to a stop and waiting at a stop light, the pump will be off.
It was a quick fix to tap into the break pedal switch and use that to turn a relay which applies power to the pump. Saving power and noise -- wahoo!
As many have noted, the power steering pump is most useful when at a complete stop and not needed when traveling at speed. So one of the next goals is to add some logic to the system to disable the pump above 25 miles per hour. All the wheels have speed sensors, so I should be able to tap into those and get something working.