There is more to DOD than just shutting off the fuel injector and spark; a solenoid operates an oil circuit passage which disables the valve lifters on one half of your engine, so the intake and exhaust valves remain closed on one bank of cylinders. The pistons in those cylinders just compress the air trapped in the cylinders over and over, using the air as a "spring" to force the piston back down the bore after each compression stroke. Additionally, the throttle blades open to allow more air to the cylinders which still operate normally. DOD actually works by opening the throttle blades during low-load, low-throttle conditions, reducing pumping losses on the intake side of the engine (you would see it as lower vacuum in the intake). IOW, the engine is not having to work as hard to suck air past nearly-closed throttle blades so it takes less fuel to produce a given amount of horsepower.
The ignition coils, fuel injectors and throttle body could be controlled by a computer (or tuning) upgrade, but I dunno if the pre-DOD engines can be retrofitted with the hardware to disable the valves. If they could, I agree the EPA (or Congress, or somebody) should require GM to sell the kits at cost or cost+1%, and have dealerships install 'em at a low set rate, or give equivalent tax breaks to consumers who agree to retrofit.