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Though, Detroit probably should have pushed on it more because:

I would favor the development of three other technologies:

Variable-displacement engines
Power control using variable delayed intake closure instead of throttling (gives the Miller Cycle's advantages at lower power levels, while allowing more power when needed)
A five-cycle mode of operation when cruising (another freeper posted a white paper here some time ago; on an inline four, during low-power operation, the inner two cylinders would be used together as a double-sized "compounding" cylinder for the outer two).

I would think these would improve efficiency as much as would hybrid technology, but at lower cost.

1. Variable-displacement engines

Mechanically complex (but then, compared to a hybrid . . .)

2. Power control using variable delayed intake closure instead of throttling (gives the Miller Cycle's advantages at lower power levels, while allowing more power when needed)

This sounds better than variable displacement.

3. A five-cycle mode of operation when cruising (another freeper posted a white paper here some time ago; on an inline four, during low-power operation, the inner two cylinders would be used together as a double-sized "compounding" cylinder for the outer two).

I could understand wanting to expand the exhaust gas of cylinder #1 "isentropically" in cylinder #2, but isn't an exhaust turbine more practical? As long as we're discussing hybrid, why not consider an electric generator to absorb the energy from an exhaust turbine?
And much as I dislike the smell of diesel fuel/exhaust, I have to admit that compared to a throttled gasoline engine the diesel cycle is reasonably efficient at partial load.
For the payoff you can hope for, using hybrid to tame the dual-fuel diesel cycle really seems to me to be worth a look. Its main fuel isn't fuel oil, so the smell would be less obnoxious. And since its main fuel is carburated, you get smooth, blue-flame combustion rather than normal diesel knock.