"As an example the Forumslader has 5 stage load matching, which is switched by a microprocessor, plus a voltage booster at low speeds to maximise output. "

Trevtassie, Is it possible to post a reference to the source for that? The most complex of the circuit diagrams I found for a version of the Forumslader had two values of serial capacitor, controlled by a relay. I wasted some time looking for more.

"Dynamos are constant current, usually around 500mA. "
If that were strictly true, it would be easy to extract more power. A frequency-dependent voltage source in series with resistance and inductance seems a better, and physically plausible model. However, I measured an almost 2:1 variation in inductance with wheel position (Sturmey-Archer GH6 Dynohub).