Sturmey Archer makes a 3 speed hub with a band brake mount on the left side. I'm pretty sure the thread on band brakes use a standard English thread, so you could theoretically thread a track cog on there and use it as a flip flop. Chain line should be within reason.
No way to posistively lock that cog on there though, so I wouldn't be relying on it as a braking device.
http://www.sturmey-archer.com/produc.../id/40/specs/1

All of this is really uneccessary though as Sturmey Archer has now released the 3 speed fixie.
http://www.sturmey-archer.com/products/hubs/cid/3/id/47
It will take a screw on freewheel cog or a Shimano freehub splined cog to make it fixed.