Yeah, I've got a 6-bolt sprocket on my fixed gear. I think it's a good way to go - cheap, plenty of wheels about, good braking, more future-proof, and you could put gears on one day if the frame has a derailleur hanger.
I don't have a rear disc but that's an interesting way to do it for sure...
There's a CXC on radavist with 6 bolt cog on the disc mount and problemsolvers 6 bolt adapter for the disc brake on the freehub body.