I used this several years ago for sourdough but thought it might apply for capturing wild yeast. This was used for wild sourdough starters but I think it would also work for beer starters. If you search for wild yeast sourdough starter there are lots of hits but I think this is the person that started it originally. http://www.thefreshloaf.com/node/233
This the recipe I used.
1 cup natural pineapple juice
1 cup whole wheat flour
Let sit in an open window, covered by a thin sheet of cheese cloth.
Wait for bubbles, then refrigerate for a week and use as sourdough starter.
I thought the concept was make the medium a bit more acidic to promote yeast growth and slow down mold growth?
I will have to get this growing again and post an update.