How early did the idea of pitching brettanomyces occur to someone? Almost instantly:
The holder of the patent is N. Hjelte Claussen, the scientist who discovered brettanomyces at Carlsberg. I'm going to go ahead and hazard the observation that the Wiki site on his strain--brettanomyces claussenii--is wrong; it says he discovered the strain in 1904. Nice trick securing a patent on this technique a year before he discovered the actual strain.
I'll hazard a second observation as well: Claussen's scheme to pitch brett into lagers doesn't seem to have taken off. Sour lager (sauerlagern?)? Someone should give that a shot--there's a gold medal at the GABF waiting for the first brewery to perfect it.
It didn't really pan out for English beers, either, though you could credit Claussen with amazing foresight. It only took commercial breweries a century to start pitching brett after he suggested it. Wonder if that patent's still good?
Update: In comments Ron Pattinson writes:
It didn't take 100 years for someone to pitch brettanomyces. Courage were doing it with Russian Stout in the 1970's and probably started a good bit before that.
I've also an East German Porter recipe from the 1950's that says to add brettanomyces for secondary conditioning.