I was on this truly great site (http://www.online-literature.com/) that houses all these works (including articles) by great authors AND that has a really decent search function. You can check almost all of the works of the authors they have to find specific references.
Twain refers to sacred cows often, but only in the literal sense. He never refers to hamburger, and - out of all of his printed material – of course, the quote is not to be found. I still believe that hamburger was in use to refer to minced meat (my Saff term) before the time of his death, and, that if he had uttered the words, it would have been this and not the modern hamburger he would have been referring to. I also believe that sacred cow was quite possibly used metaphorically before that time (as suggested by my previous posts), but I’ll admit that it’s unlikely he coined the semi-proverb (I abhor the dastardly, dishonorable bastards who assigned it to him!). I admit defeat.
And I can’t live with the guilt anymore. I tried. I’ll change my signature accordingly.