Alan - The idea of cutting back cylinders that are doing the work is sound, like that 8-6-4 idea that Cadillac had. I'm not sure what the problem was with it, but I think it didn't catch on too good. It was a little before my time.
The problem I see with the diesel is with the compression stroke. If there were some way to open the exhaust valve during the compression stroke I think it would work great. Otherwise, a significant amount of work that the, "running" 3 cylinders are doing would be solely overcoming the compression of the, "non-running 3. " Maybe there's a way around it, but I don't know of any with fixed cam engines. Just wait until we start getting those other valve actuated engines (solenoids?). Then it would/will be just a program to do anything you want with the valvetrain. Smart valves. Full open at higher rpms, less open at lower rpms. And for high effieciency, just a crack open.
Besides, wouldn't running on 3 cylinders create a balance problem? I'll have to study it but, seems like you might beat the crank to death by loading unequally (through hundreds of thousands of revolutions running on 3 cylinders). Mazda went through a lot of study on stuff similar to this with the Wankel. Since its so small and the fuel consumption is so poor, they were trying to improve its efficiency.
Sorry to go off here. Maybe there is a way to overcome the energy it takes during the compression stroke. It would work great then. If so, let me know.