I think it may have been me who mentioned the Imagineers' method of aging brass. I have scoured the Internet, but I have not been able to find any references to the recipe. My vague recollection is that in addition to cow poo it included straw, vinegar, and molasses. I could be wrong abnout the other ingredients- it's been many years since I got the tour of the "underside" of Epcot Center.
In ye olden times (and in 3rd world countries) tanning a hide to make leather used dung, urine, cow brains etc. and it sounds a bit like the process you have just followed. So perhaps modern tanning techniques would age the brass as well. I found this crowd (with an outlet in Hunstville).