Instead, you need shunt regulation.  Some motorcycles come with permanent magnet alternators and the voltage regulator in the stock design is typically a shunt regulator.  Harleys, for example.  Depending on the year of the bike, it might have single-phase or three-phase rectifiers.  Anyway, if you look for a stock regulator from a modern Harley with a high-amp electrical system, you might be able to use that with your DC-520 and avoid all the headaches.  I haven't yet done any searching for specs of the bike voltage regulators, but from looking at the dc-520 I thik it puts out less power than a modern Harley alternator.  
Even into a dead short, the dc-520 maxes out at only 25 amps.  That's less than what the typical late-model Harley alternator is rated to put out (and not into a short, either).  
One caveat -- the bike regulators are designed to have a lot of air flowing over them.  You'd have to take that into account when considering thermal management issues.