Turboshaft expansion machine from turbocharger?

[Blunt Eversmoke]

This is somewhat off-topic, but best fits here.

On a thread of a Russian forum, free piston gas generators are discussed in great length; it seems that it is possible to build one of the non-balanced, asymmetric variety with combustion chambers on both ends pretty cheaply and easily once the calculations are done right; for power control, one just makes multiple of these and starts or stops them depending on the power output needed.

Now for what to do with the generated working gas:

Actually, turbines as expansion devices start to be economical if slow-running, so for a small-power unit (under 250 KW), it seems that a volume expansion machine in the style of a steam engine or a Gerotor expander or somesuch would be the best choice.
However, regulating the cut-off depending on needed torque and the inlet timing depending on RPM is not trivial at all. Also, as to the Gerotors, units capable of using hot (400-600°C) gas that are also resistant to pollutants such as ash and soot seem to be pretty rare and pricey.

So, to construct a proof-of-concept small engine cheaply and with less hustle, one would have to use a turbo, after all.

It has been tried by some here and by many others to take just the charger section and drive it with a belt to create a centrifugal supercharger. For this application, it could be tried with the exhaust section of a turbocharger, as well. What exactly does it take to make such an arrangement? Bearings, pulleys and belts, assembly?

As an alternative, are there ready-made belt-driven turbos on the market? Being made from the cold-air sections of turbos, would their turbine wheels cope with 400-600°C of heat (often, the compressor sections of turbochargers are made of less heat-resistant alloys...)?

Thanks in advance!

EDIT:

Using the blowdown turbine section of a turbocompound truck would be the best since it would provide a reliable reduction gear, but I don't know how costly that would be, and there's zilch on German, stateside and Kingdom ebay, so there's no way to find out.