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Exhaust brakes work awesome for quick warm ups. The newest diesels use the VGT to create a ton of backpressure (upwards of 40 psi in the exhaust manifold at idle) during warmup to get things hot fast in order for the aftertreatment systems to work properly.
Exhaust heat exchangers, AKA EGR coolers, must have high coolant flow through them at all times. The idea of one of these being utilized for quick warm-up is sound, but you would have to bypass the exhaust once up to temp instead of valving the coolant. The instant the coolant stops flowing in an EGR cooler (at least when exhaust temps are high), it flash boils, turning the heat exchanger into a paper weight. I've seen it a hundred times when diesel engine loses its water pump belt while pulling a hill. The EGR cooler basically erupts and the engine inhales the entire contents of the cooling system through the intake. Not fun for the guy paying the bill. Fun for the guy fixing it (me).
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