That was me. More early refs.:

Chester [Pa.] Times (Dec. 15, 1904):

                A ROCK CANDY MOUNTAIN

Dear Santa Claus: I want a rattle a doll baby and what ever you do take me to rock candy Mountain and lemonade springs. My name is
                                  Willie Irwin
                                  110 East 3d street, Chester, Pa.

Owhyhee Nugget (Silver City, Ida.) (March 3, 1910):

“This is the kind of weather that makes a person feel like singing tat well known and famous old song: ‘Oh, the birds and the bees and the cigarette trees and the old Rock Candy [sic] mountain,’ etc.”

Detroit News (June 18, 1911):
“They resemble a hobo’s dream of the Rock Candy Mountain on a frosty morning.”

Coconino Sun (Flagstaff, Ariz.) (Oct. 10, 1911):
“Joe Barr of Rock Candy Mountain near Seligman has been annoying his friends here for several days past.”

McClintock’s recording of “The Big Rock Candy Mountain” was released in 1928.