I sent the cow/horse poop suggestion to my son (potential steam punker) and wife (former (perhaps perpetually) farm girl). My wife provided this response:
Fast-tracking patina for brass: (from Mom)
Left on its own without any intervention, unlacquered brass will begin to darken in about a month and will continue to take on a deeper patina with age and use. (Look up the next time you are seated at our dining room table.) To accelerate this process, placing the items in HOT water will speed up the oxidation. Another trick is to place an open can of traditional latex paint next to the unlacquered brass item in an enclosed area for at least 24 hours. The fumes from the paint will quicken the patination process.
And, by the way, BRASS IS BACK, in a big way. The appetite for nickel and chrome is at the saturation point. Just ask Martha Stewart.