Good question, I've never heard of this though I can see if the vehicle is
really antique than a simple DC motor run in reverse could have been used to provide a charge, just with no real voltage or current regulation.
Using regular permanent magnets (in the case of a regular DC motor run in reverse) would not be a very good solution because the voltage output would be directly proportional to the RPMs.... about the only control one could use on a setup like that is a cutout relay to break the connection, but this would only open if the "generator" was not putting out enough juice, and depending on the speed of the engine, the voltage could be very high!
Some early generators used a third brush to provide 'static' voltage regulation; that is, it would be manually adjusted to charge at an optimal RPM, but even in this circumstance the third brush still relied on a field coil to self regulate.
My thoughts would be that although this is theoretically possible, it is not practical and is most likely a wives tale or a story that got started when a farmer 'improvised' something to keep his tractor running!
Otherwise, the vehicle would have to be
pretty early to use something like that!
