I've seen people mention pulling a trailer on their fixie in this forum. I left my geared bike at work last week and needed to pull my daughter across town so I bolted the child trailer hitch receiver on and gave it a shot. Here's a photo of the rig.

My city has some short, moderate hills. Pulling a 50 pound child obviously results in slower starts and stops. One one downhill, I skidded about 30 feet past a driveway I needed to turn into. I fishtailed a lot too, which surprised me.

I rode 28 miles pulling the trailer; by the time we returned home I was pretty tired. On an uphill close to home, I counted 0.29 rpm -- that's one crank of the pedal every 3.5 seconds. My 9-year-old son, who was riding with us, kept telling me to hurry up and pedal faster.