@ Tom: Wild yeast native to fruit skins generally isn't Brett. It tends to be strains of Saccharomyces Bayanus among other things. You'd need to do a lot of work to isolate just one strain of wild yeast from fruit skin.
Brett is native to many fruit skins - maybe not all fruit, and maybe not as populous as other strains, but still present. It would be fairly easy to isolate a single strain, you can just plate it and streak for singles. The trick to me is how to tell brett from sacch using things your average homebrewer has access to.
The good news is that most wild yeasts croak out at low ABV, so S. Bayanus and/or Cerevesiae have an advantage since they can stand higher ABV levels.
How are you differentiating between S. bayanus/cerevisiae found on the fruit skins and "wild" yeast?
I also wonder if it wouldn't work to sort of 'interbreed' a wild yeast with a small culture of lab yeast that has the characteristics I'm looking for. But given that they produce by budding as opposed to Missionary-style, this may not work. I Am Not A Microbiologist (although I play one on TV!)
This could work, but would be pretty hard to do. The wild sacch strains should sporulate without a problem, but the spores will tend to mate with other spores of the opposite mating type. The way you would do it in the lab is to knock out the mating type switching locus so that each haploid mother would produce daughters of only one mating type. Then you can sporulate and dissect the tetrad and get separate colonies of both mating types. After that you can cross them with spores from other strains.
However, getting viable isolates from a beer strain is even more difficult. Most of the strains I've tested don't form spores at all. Of those that do, none had viable tetrads. Some of the spores would grow, but never more than 2 of the 4. So you'd have the same problem as above and need to knock out HO, but you've also got to get lucky on the spores. In some cases it just might not be possible. Most beer strains exhibit aneuploidy, meaning that they have different numbers of each chromosome - so they might have 2 copies of chromosome 2, 4, and 12, but 3 copies of 5, 6, and 14, and just 1 copy of 7, 8, and 9, etc. Beyond that - even if you got viable spores, there is no guarantee that the genes would segregate such that beer made with those spores will be any good.
In general it would be much easier to stress the yeast or use a mutagen to alter the genes at random, then select isolates and do test ferments and find one that you like. It would be tedious, but I think it would have a much higher chance of success.
So where did a brewery get it's yeast back in 1850 or so? Did they just use wild yeast over and over again for a few hundred years until it mutated into something that made good beer?
You have to go back a lot farther than the 1850s, but yes, it was more of a trial and error thing. Beer was often fermented in wooden vessels which would harbor yeast. If a vessel was known for making bad beer you'd get rid of it and make a new one. Going back, brewers may have used a stick to stir the wort, and the stick would harbor yeast too, effectively inoculating the wort.