My Swiss friend Vincent Delay sent me a brilliant letter in response to my May 16 blog post "Who, then, is Porlock?" in which he attempted to answer the question posed in the title. It's worth posting here in full.
I just read your news about “Porlock” on http://bakerstreetbeat.blogspot.com/ . I also had the pleasure to get The Adventure of the Murder on the Calais Coach and I began to read it.
Using the Master’s own methods in my Watsonian way, I reached the following conclusions regarding the identity of “Porlock”.