If I were to buy a pile of those IRD Defiant 5-7 speed freewheels and use the cogs to build my own freewheels, they have 13-21 then the 24, 28, 32.

Then Sunrace has a 22T cog in it's 5-6 speed freewheels, would it be compatible with the IRD? http://www.sunrace.com/userfiles/products/specs/MFR.pdf

Sunrace also makes a 7-speed freewheel that includes the 23T cog. Sunrace R30 7-speed 13-25t Freewheel from BikeBling.com
Or I could use this Sunrace freewheel to start off at the 12T Sunrace R63 7sp cassette, 12-24t from BikeBling.com

So now I could go from 12-25 in single tooth steps. Not bad.

Somebody like Velo-Orange or Rivendell or Rene Herse might have the ability to get a 3-speed freewheel made. I've used the Surly Dingle on a SS/FG conversion I did with an old Raleigh Sprite frame and it worked very well. Adding one more cog shouldn't be that difficult, assuming there's enough demand. I think Sheldon Brown had an article about connecting a 3-speed cogset to a SA hub. Bianchi had a bike model not too long ago that did something similar; it gave the rider a lot of gearing they could select using only one hand.