This was my first attempt at retro-direct, and for the most part it turned out pretty well. I used the techniques here. One thing I wish I'd done is set up a larger difference in gear ratios. The two free wheels are 20 (backward) and 16 (forward). Also, the 20 tooth was pretty noisy. Not too bad, but it sounded like a fly was buzzing in my ear when I pedaled forward. Here's the method I used to build the hub if you're interested in trying this out. Pedalling backward uphill is a little strange at first, but starts to feel almost natural once you get the hang of it.
Here's my second iteration. This time I included a second chainring, so now I have a 4-speed retro-direct! Biggest difference here is the second tensioner.
Unfortunately because I mostly buy cheap/used parts (this whole experiment only set me back about $300, most of which was for rims, spokes, and tires), that first frame broke. So, I got a new frame; hence, the third iteration. It's basically the same, except the second tensioner pushes the other way.