I have no doubt that some regulators have one-way polarity.

It looks like the original 1118825 regulator had one-way polarity. However, that is a 12v regulator.

http://www.worthpoint.com/worthopedia/1953-1956-cadillac-voltage-regulator-138741167

Just because I like documentation, it would also be nice to see a mid-50s 6v alternator document.

And, just as Denny noted Kieth Hardy has kindly provided that information for some 6v electrical equipment - see page 4, BULLETIN 1R-111 12-1-48 (DELCO-REMY Electrical Equipment Service & Operation Manual Operation and Maintenance Handbook DR-324).