As someone whos studied mosses indepth in college it is really odd to me why the buttermilk idea works.
I challenge that the buttermilk has nothing to do with improving the moss growth, but rather acts like a thickening agent to allow for the slurry to better ahere to verticle surfaces. Most mosses need an acidic environment. This is necessary for the sporophyte coating to become weakened by the abundance of H+ ions and thus allow the release of the gametophytes (what you see as "moss"). Buttermilk is basic and will help prevent this from happening. And since what is growing in most of these moss mix concoctions people make up is generally the new gametophyte from a spore rather than the old "parent" one that was blended into oblivion, it would make sense to use a medium that doesn't inhibit growth.
The one way I can see this working, is that the buttermilk is broken down by bacteria and lactic acid is produced which then gives the needed H+ ions. But that will smell bad, and possibly allow for fungal growth to take over and then the moss doesnt stand a chance.
My adivice, don't use buttermilk and find a better, more inert substance, with a low pH.
Like um CLAY!!!! I challenge anyone that if they took a mixture of clay, loam, and water and mixed in their moss spores, smeared it onto something and kept it wet, you'd get just as good, if not better results than the buttermilk/beer/milk/fertilzer/whatever wives tale addative. Minus the smell and ruining your waterfeature.
Just a botanist's perspective.
I've tried the beer and buttermilk trick with little sucess. A few sprouts here and there but the smell was not worth it. Plus I had to do a full water change on the one below to get the darn thing from stinking up the house. It wasn't until I later tried the above stated method that I have been getting excellent results ever since.
Oh and to back this up: