I had a fix gear made that does much if the OP's wants. It has a very long dropout and can handle from a 12 tooth cog to 23 without messing with the chain. I ride it with a 144 BCD Sugino 75 crank (for its track quality roundness and very low Q-factor that my knees love), usually on a 42 tooth chainring. This gives me gears between 49 inches and 95 inches. I use double sided fiz-fix hubs and carry a cog wrench on the top tube for very hilly rides.

My Cycle Oregon ride up to and around Crater Lake went like this: Left camp on the 17. When the climb got serious, flipped the wheel to 23. At the rim, I swapped the 17 cog for the 12. Rode around the rim on the 12 and 23. Down toward Prospect on the 12 for miles. When the hill leveled out, I took the 23 off and rode into the new camp on the 17 like I started. Spent very little time in the wrong gear. Did have to do 6 flips and 2 cog changes.

Edit: I too got thrown off by the word "singlespeed" and wondered the entire post why the OP was going with a cassette if he skipping the derailleur. But given that I created my first SS by simply taking the derailleurs off, putting the chain on the gear I wanted and shortening it, I didn't question what the OP was doing.

Ben