I don’t think this is immediately available. When we were developing Josekle, we had a similar desire to directly access an SGF dump of the OGS joseki dictionary. However, we wound up scraping the website somehow to build an SGF file that we then converted for our purposes to the make the Josekle game.
Maybe some of the code here might be useful: josekle/sgf at master · okonomichiyaki/josekle · GitHub
There is even a cached SGF of part of the tree, however, it does not contain the various commentary.
Actually, I don’t really understand that (joseki dictionary scraper) part of the codebase, as I’m not familiar with ruby and it was solely worked on by @okonomichiyaki.