How 'bout this: mount a clamp-on friction downtube shifter very low on either the downtube or seat tube, and cable it up to an actual derailleur. From the bottom of the downtube, it could route normally under the BB with a guide; from the bottom of the seattube it could either loop around the front of the BB and then through a guide, or could probably be creatively rigged up to act direct from the shifter to the FD. Wouldn't be much kludgier than some rigged "suicide" shifter pictured earlier, would be totally impractical to use in motion, keeps the handlebars and most of the rest of the bike clean of shifting detritus.