just to throw out .... but there are a number of state extension services that have directions and discussion for spalting wood. Most of them talk about burying the wood under leaves (I guess to both preserve a little heat, and supply the spores. it might be worth reading some of what they have to say before you start. I never would have thought of beer and yogurt though (yogurt culture is a bacteria, and beer is a fungus (yeast) - so beer might give you the best results???
HOWEVER, beer like the standard Bud, Miller, etc. is highly filtered and sterilized - and so will not supply the needed culture. many of the domestic craft beers are not so highly filtered, and will supply some yeast. You would likely have the best luck with a "wit" style beer (the "wit" meaning "with" - really with yeast. They are not filtered at all, and should be hazy with yeast.
If you really want to go for broke, use any good imported Belgian beer. They are not filtered, and are just chock full of goodies from the local environment there. I personally am really partial to Belgian Kriek. You should be able to pick it up at any good liquor store......