Regulating a steam-powered model generator

Hello all. Thanks for taking up your time to read my post.

In this age of questing for ever-more-efficient circuit designs, I have been presented with an anachronistic design problem: I need to keep a roughly constant load on a steam-powered turbine generator as the datasheet of http://www.kynix.com/uploadfiles/pdf9675/UBA2024P2fN1_8392.pdf. So I am looking for a linear regulator circuit that can dissipate slightly over 50W when all of the system electrical loads are switched off. And this is in a hostile environment: It's attached to a model steam locomotive, so exposed to relatively high ambient temperatures.

I presume such circuits were once commonplace (for example in early automotive applications... or in real steam locomotives!) but I am not finding a lot of reference information on them online. Can anyone recommend a good design resource for such circuits? And perhaps an existing linear regulator part with a scheme for dissipating that much power in a high-temperature environment?

The raw output of the generator is specified to be 3-phase AC, 0-12V (RMS? Peak-to-peak?) and 0-4.2A.

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