I've made sourdough starter with organic whole-grain flour, as well as one with rye flour. Here's how I made it:
- Mix the flour with water.
- Cover and let it develop for 12 hours in dark, draft-free place.
- Remove half of it, replenish with fresh flour and water, then back to step 1.
- Once the starter double's itself every 12 hours you start replenishing with white flour.
- After yeast is a week old and still going strong, it's safe to use.
Like with the grapes you mentioned, this works because of the natural yeast from the fields it was grown in lives on the wheat or rye. I would be willing to bet that similarly, yeast lives on the barley, however, since the barley used in brewing has been malted all of the yeast is killed off.
I think you'd have to get the yeast using unmalted barley or organic barley flour, and along with it you'd probably pick up some Lactobacillus as you do in sourdough. This would give you that distinct sour flavor by producing lactic acid during your fermentation.
Also, as you mentioned, the vigorous boiling will kill any yeast off, so you must figure out how to: introduce barley and its yeast to a prime growing environment, crowd out any other bacteria that might be living on the barley, stabilize the yeast strain, and pitch it after your wort has cooled to room temperature.
I would just get a yeast culture the same way you do with bread (of course being more carefull and sanitary):
- Mix some finely-ground barley/organic barley flour (unmalted would render the best results I imagine) with water (maybe some liquid malt extract as well to feed the yeast?).
- Put in a sanitized container with an airlock or in an air-tight container and let it develop for 12 hours in dark, draft-free place.
- Remove half of it, replenish with fresh finely-ground barley/organic barley flour and water (lme?), then back to step 1.
- Once the starter is going strong (lots of CO2 and activity) you start replenishing with malt extract only so you don't introduce any more foreign organisms.
- After yeast is a week old and still going strong (and doesn't smell funky), it's safe to use.
You pose a very interesting question and I am very tempted to try this myself. It works with bread, why not beer? I think the key is going to be to get the yeast culture up and going long enough to crowd out any other organisms that might be living in there. If you find a good strain, just keep it alive and reuse it!
Let us know if you try something out and how it went!