Dave, I wouldn't want to ride fixed without a lockring. I've had the Surly SS disc hub which has both threads but the design was flawed and it ate bearings for breakfast. So I thought MTFU and forget about the rear brake, and I bought On-one's excellent double-fixed 135mm hub (OK the axle's a bit short but you can't have everything) 🙂
Phil Wood's have done an interesting new double-disc hub for bolt-on track cogs but it looks like it's only in 120mm at the moment.