Isn't the freewheel aspect pretty much covered by the flip-flop hub itself? I kind of assumed it was built in.
(Mind you, I didn't build the ss/fixie flip-flop sitting here in my living room; but I've always assumed the freewheel mechanism was the "flip" part of the "flip-flop" hub. Which makes it possible to switch from ss to fixie so simply. But I am a fountain of ignorance and have never flopped it.)
Surly has some nice info: http://surlybikes.com/blog/spew/spew...d_drivetrains/ You can also call them and ask for help.
Looking at mine, I see a cog in all its nekkid glory glommed onto the hub with nothing else ('cept the lockring). It's on the freewheeling side. The fixie side is bare.