Lambic isn't open fermented, it is merely cooled in open air slowly to partially inoculate. Then straight into barrels, which have a culture that has built up from reuse.
I have caught wild yeast once in my backyard, but I used a very small starter and fed it for a couple months before pitching it into my "lambic". Didn't want to risk the whole batch/hours of work for a very possible loss. I can say that I caught some lovely stuff and I can tell you in a year what it does to my beer. The starter smelled like cherries and sourdough.