Ferrotec responded to my email in 36 minutes! Wow.
Marketing specialist Vanessa Rene at Ferrotec says that Bob Berkowitz of Acoustic Research began studying ferrofluid in 1972, using it to damp resonance of a tweeter. Fred Becker and Lou Melillo of Becker Electronics were also early adopters in 1976. Panasonic was the first Asian manufacturer to put ferrofluid in commercial loudspeakers, in 1979. The field grew rapidly in the early '80s, with Meyer Sound, Apogee Sound and many more jumping in.
Other archival sources show that Dana Hathaway of Epicure in Massachusetts was using ferrofluid for tweeter damping in '74, and he noticed the cooling mechanism.
By 1979, Showco was putting ferrofluid in woofers for the cooling effect.
https://books.google.com/books?id=fAEAAAAAMBAJ&pg=PA61