How about using a custom dictionary for your project (as described in the RStudio blog post you linked) that you keep under version control? It’s not very hard to make a Hunspell .dic file — at its simplest, it’s just a UTF-8 text file with one word on each line.
Btw, the Hunspell integration is provided by the RStudio IDE, not the RMarkdown package, so you might want to recategorize your post so it reaches the most relevant audience.