Yeah, that's my thinking. And I've seen designs that don't look too bad.
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.
Agreed. This seems like it really shouldn't be too hard, but could offer some big improvements when operating at moderate throttle (which is what most engines spend most of their time doing). The biggest difficulty, from what I understand, is that there needs to be a certain volume above the piston at TDC between the compression and power strokes; that same volume must be there after the exhaust stroke. Consequently, if air is drawn into the engine and then some is pushed out, this will cause some exhaust gasses to go out the air intake.
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?
How about using two turbines in a jet configuration? Actually, there are some variations on that concept which would seem like they could yield some very good effiencies, but would unfortunately be rather expensive.
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.
I'm partial to the notion of continuous-combustion engines, myself; it would seem that going with continuous combustion would allow one to do things like put a catalytic convertor between the combustion chamber and the power-producing area (be it a piston, turbine, or whatever); this in turn would allow some of the heat produced by the cat to be converted into useful work.
There was a time when I thought that recovering the chemical energy that you describe mattered - but when you realize how low a fraction of the energy in the fuel goes to the catalytic converter I think it is pennywise to chase after it. Can you actually make your cat stable at the peak working temperature of the cycle - and do it cheap enough to make a profit from recovering 1% or so of your fuel's heat value???