The thought of using a system where the axle or BB has to come loose to allow the change pretty much fills me with dread. Reading the procedure for that Campy system it is obvious that this is not something you want to be doing in the heart of any downtown traffic. Not to mention that with a fixie the need to maintain the pedal cadence would make all that balancing and pedalling while maintaining a neutral chain load pretty tough.

What about a derrailleur like shifter/tensioner that actually locks up solid between shifts? There would need to be a way to unlock it then shift to the desired side and shift the chain with it and then once in place to relock. The rider would need to plan the shifts such that they did NOT suddenly have to back load the pedals during a shift or the obvious bad things would happen. But I can see where this could be done using some manner of a control pod on the bars that declutch for the first portion of the lever or twist travel and then shifts and then allows for relocking as the control is released. It would likely be a longer travel than usual due to the added function. Much like upshifting two gears at a time with the Shimano MTB rapid fire pods or a double shift with a road brifter. For shifting down to the smaller rear cogs the throw would be more like a single upshift to de-clutch and then have the normal small click at the end of it.

The other and far more ideal option to my mind is that Sturmey-Archer fixie 3 speed hub mentioned above. It would look a lot less Rube Goldberg like than any sort of external changer that deals with shifting chains and multiple sprockets.