[Charles Martel]

Was that like the ZR1 that you could put into valet-mode and lock down half the engine?

Yep, I think it's the same idea. The ZR-1 used a separate key switch for that, though. Speaking of the ZR-1, the LT-5 engine's successor - the Cadillac Northstar - first reintroduced GM's variable-displacement technology. GM didn't use it to boost fuel economy, though. Instead, it used the system to cut off fuel to cylinders (up to 4, I think) and pump air through them as a "limp-home" cooling system.

[IllumiNaughtyByNature]

With all the technology in engines now, the ability to regulate fuel efficiency is way past its due.

Its industry dragging its feet IMO.