sorry, i run a 30tooth...
finding two sprockets and welding them to a left hand drive fixie cog should be pretty easy.
while i'm interested in this experiment, i'm pretty confident that a multi-speed system, based on a derailleur bicycle system, will fail.
there's too much torque, problems with chain alignment, and chain tension.
i think the only way it would be functional would be to completely re-build a bicycle system on a grand scale. i don't think any derailleur made today will work, otherwise you would've seen one on a moped. i'm sure every old moped manufacturer has tried, tested, and failed this idea 40 years ago.
bicycle derailleurs are fragile objects. just talk to any hardcore Mountain biker or roadie and they'll tell you. mountain bikers have problems with "chain slap" and other related drive train problems on a routine basis.
it's just asking too much from what we're working with.
the only thing i think will work is an internal hub, or an external, bolt on transmission.
all that being said, i'm still interested in the final outcome of this.
but, even if it more or less worked, i don't think i'd wanna shift gears at 40+mph. it might work at low speeds for climbing hills, but i don't think it would work for speeders...