[dcb]

I could see a momentary switch on the wheel or shifter that "locks in" best bsfc, once that is determined (using existing cruise control actuators and safties possibly).

Then you can just clutch/shift like normal and hold the button once you are re-engaged until it is time to shift (see indicator light thread). Could also work on an automatic for most efficient accel.

It needs to be bulletproof for general consumption, but I'm sure a wary hobbiest could pull it off.

My TDI doesn't really need one though, the most efficient curve is basically the torque curve, you just have to wang it open and shift at the right time, and stop accelerating at the right time.