I've done a lot of work on things like this. Sadly, there is no specific instance. You've identified the use of the term for literature, and it grew organically in use in small publications following that time period (very early 1910s) until it was a generally accepted term. However, in the case of speculative fiction, it is not used to denote "worthy of being read" as in older literature (Conrad, Melville, etc) but to denote clear continuity.
Early writers it DEFINITELY was used to discuss include:
Lovecraft - (his work, vs. the work of others, led to what can only be called a CANON vs FANON (a fun portmanteau for "fan canon") argument.
Tolkien - There are many who feel that Tolkien's "canon" does NOT include the Hobbit, as it was written before his histories of Middle Earth were invented, and The Shire is sort of just tacked into the middle of that universe for the sake of a relatable protagonist or two. The Silmarillion is widely considered the de facto canonical source.
That said, my understanding (which I can't source currently, my Google-Fu is inadequate) as I recall from my years of literary study, yields that several "speculative fiction" novels have long been used as arguments against the actual Literary Canon, including George Orwell (1984), J.D. Salinger (Catcher in the Rye), Kurt Vonnegut (Slaughterhouse Five) and other writers from 50+ years ago.
Hope this helps!
What you already know IS the closest thing to the correct answer. Even though it's not definitely Sci-Fi/Fantasy, Doyle has elements of the preternatural, and many pulp novelists emulated him. One imagines that if Mary Shelley or Bram Stoker had been more prolific, someone may have applied the term to them as well, but it didn't become a relevant term until the age of the serial.