At part load I think you'll be hard pressed to find enough power that can be converted, as I said already...
The difference between a turbocharger turbine and a turbine that is supposed to collect as much waste pressure as possible is a turbocharger creates a greater pressure drop in order to generate more power.
Accessories using 20hp? Maybe at full speed, full load, but again, normal driving happens at extremely low load and rpm, where there is almost no wasted kinetic energy in the exhaust to speak of. When you're at part load, there isn't enough pressure leftover in the cylinder when the exhaust valve opens to drive the accessories, but there is far more than enough heat energy. Say on the highway you are using 20hp. The engine running at part load is throwing away around 30hp in the exhaust (the number doesn't really matter that much, it's just important to note that it's greater than or equal to the amount of useful work you're getting and on the same order of magnitude, because the ideal cycle at lowered compression ratios dumps half the fuel energy into the exhaust, throttling loss goes into the exhaust, and cooling/friction loss goes on top of all that).
Say you had a pathetically small heat exchanger for cost/weight reasons, and a single stage turbine running at 60% of ideal efficiency which I believe is a bit below 70%. The turbine gives you back around 35% of the available steam heat energy after all conversion losses, and say the steam only carries away 10% of the available waste heat. At 3.5% conversion you are still getting over 1hp from the waste heat, which is enough to cover all the electrical loads plus a water pump and then some.
Although if I may note, the Subaru electric turbo is a great thing. A normal turbocharger is attached to the compressor, and has issues with transient response and such. The electric turbo breaks the turbocharger into 2 components, a turbine which only needs to absorb excess energy in the exhaust instead of creating backpressure for more shaft power, and a supercharger which can run off the battery for a short period to improve transient response. When you don't need the power, the engine behaves like a naturally aspirated engine with enhanced exhaust expansion via turbine
