(I'm assuming here you mean spontaneous fermentation, used originally in belgian lambics. Open fermentation is the practice of fermenting in an open vessel but in a controlled environment, not allowing other microorganisms to get in, like Anchor and Sierra Nevada still do these days, and sometime ago very common in the industry.)
You will probably never get a definitive answer to it but based only in my theoretical knowledge I would say terroir is not so determinant as people use to think, even for spontaneously fermented beer.
My first guess is that, aside from very extreme/sterile climates and landscapes, microflora in the air is pretty much the same, at least in terms of variety and the species that play a main role in spontaneously fermented beer, which aren't much more than a few (acetobacteria, sacharomyces, lactic bacteria and brettanomyces). I know, you can find hundreds of different bugs in a single lambic, but from all that, only a few really play an important role. You can see this study here from UC Davis, based on Allagash's Coolship.
One thing that supports this argument is that american breweries are successfully making lambic-style beers following almost the exact same traditional process as the belgian. But, then, one thing they've already learned is that one of the main factors in the early process of inoculation and fermentation is temperature, what implies year's season as well. Usually they leave the wort to be inoculated over night at fall. You can listen to what people from Allagash say about their Coolship Ale, for example.
The rest of the process, well, now it depends on infinite factors (barrels, wood, storage, temperature, etc). But, your question was about distance between places, what suggests the terroir aspect of the production. So, I would say (and that's a completely wild guess), since you're in a place surrounded by any vegetation, with a minimum of biodiversity, and have wind, it doesn't make a big deal of a difference.
Oh, and don't forget one thing. You get (if so) a consistent and regular flavor from those kind of beers only because people blend it before releasing, once different barrels even of the same batch turn out to be completely different one from the other. So, again, that's why I think there isn't much sense to talk about terroir. It's not like grapes are to wines.
PS1: Off course, as climate does play a crucial role in the process, it doesn't surprise me people are doing this only in the northern hemisphere at temperate climates. I guess it wouldn't work out very well in a tropical country, which doesn't have well definite seasons, although spontaneous fermentation can be done anywhere, you just can't guarantee what is going to come out of it.
PS2: If you are heavily interested in the topic, as I am, I recommend that you listen to Savor's Salons about barrel aging and blending, and sour/wild beers, available at Craft Beer Radio. There has been a lot of talkings about the matter in the last years. Really interesting.
