Intriguing that you came up with that citation, tsuwm! Now I'm curious as to where the brass in the term came in. I'm not aware that brass monkey sculptures were a popular knick-knac at the time of the coinage, and I've never seen said item with any high-profile collectibility in antique stores or journals. And I guess it would, then, have to be a lawn or garden ornament to be subjected to freezing. I'm also equally unaware that brass is an alloy noted for becoming particularly brittle and prone to fragmentation and/or breaking at freezing temperatures. And so, embarking on my merry googling, I encounted this tasty tidbit:

The first recorded use of the term "brass monkey" appears to dates to 1857 when it was used in an apparently vulgar context by C.A. Abbey in his book Before the Mast, where on page 108 it says "It would freeze the tail off a brass monkey." [Source: Lighter, J.E. ed. Random House Historical Dictionary of American Slang. (New York: Random House, 1994): 262.]

It has often been claimed that the "brass monkey" was a holder or storage rack in which cannon balls (or shot) were stacked on a ship. Supposedly when the "monkey" with its stack of cannon ball became cold, the contraction of iron cannon balls led to the balls falling through or off of the "monkey." This explanation appears to be a legend of the sea without historical justification. In actuality, ready service shot was kept on the gun or spar decks in shot racks (also known as shot garlands in the Royal Navy) which consisted of longitudinal wooden planks with holes bored into them, into which round shot (cannon balls) were inserted for ready use by the gun crew. These shot racks or garlands are discussed in: Longridge, C. Nepean. The Anatomy of Nelson's Ships. (Annapolis MD: Naval Institute Press, 1981): 64. A top view of shot garlands on the upper deck of a ship-of-the-line is depicted in The Visual Dictionary of Ships and Sailing. New York: Dorling Kindersley, 1991): 17.

"Brass monkey" is also the nickname for the Cunard Line's house flag which depicts a gold lion rampant on a red field. [Source: Rogers, John. Origins of Sea Terms. (Mystic CT: Mystic Seaport Museum, 1984): 23.]

For the first part of the article which gives an interesting account of all the nautical associations with the term "monkey" here's the url: http://www.history.navy.mil/faqs/faq107.htm

Determined to find the brass in brass monkey...and, now it suddenly occurs to me, could it have something to do with brass balls?