We've discussed track dropouts and axle mount trailer hitches here before. Consensus seemed to be that adequate torque on the axle nuts its enough the keep the rear axle from shifting under load.
Hauling a trailer with a fixie doesn't sound fun unless it's around the block. You're adding 50-60 lbs of weight to your mass. I can feel my trailer pushing me down hills, and i don't think that would be fun with a fixie. Put the hitch on your hardtail instead.