Exhaust brakes are available for diesels to help get down long steep grades safely without cooking the brakes. They work by choking off the exhaust flow, turning the engine into a compressor. But they are expensive and trouble prone.

Gas engines have decent engine braking, just by closing the throttle valve, cutting off the air flow into the engine, turning it into a vacuum pump.

So how about installing a big throttle valve on the turbo inlet, or on the intake manifold inlet for use as an engine brake? Is there any reason that wouldn't work?