As yet there may be not much solid data, but I'm not sure you're thinking hard enough about the numbers involved. These may be competitive environments, but the sheer numbers of organisms involved, in a pretty stable environment over decades, suggests that they should be near the local adaptive peak for that environment. Where do mutations come from that haven't already been tried in that environment?
A typical family brewery is producing 100,000hl per year. Say they're pitching 6 x10^9 cells/litre, they're pitching 6 x 10^9 x 100,000 x 100 = 6 x 10^16 cells per year. Assume tetraploid, so 2.4 x 10^17 haploid genomes.
A haploid yeast genome is just over 1.2 x 10^7 base pairs, and
Dutta et al estimate the mutation rate in an unstressed, newly-hybridised yeast is 1.8x 10^-10 mutations per base pairs per division (but other papers have suggested that rate goes up considerably in the more stressful environment of alcoholic solutions, gah, I can't find it now but it's like x5 or x10-fold). So on average you will see 1 mutation per division in every 463 haploid genomes in an unstressed environment.
Which if I've got my sums right and if that mutation rate applied, means that the average family brewer's "starters" will see 5 x 10 ^ 14 mutations in one year in their first generation - and that's before you add in additional divisions, extra mutation due to alcohol stress and so on. If they were just single-site mutations that would be 40 million mutations at every base. It's hard to see where new mutations conferring significant extra fitness would come from, they've almost certainly been tried before. Obviously there's many different mutations beyond single points, but it gives you an idea - and patterns of "big" mutations appear to be non-random according to
the Dunham lab among others.
I'm not aware of anyone working specifically on UK multistrains, but the drop in sequencing costs means there's a fair bit being done on evolution during fermentation. But they tend to be on bought-in strains, which will have come out of a freezer and then had 50-odd divisions in one environment at the yeast lab before moving to a rather different environment at a brewery, so when you see mutations happening in Chico like
Large et al did, one wonders if that's not just the yeast adapting to the new environment. So that's rather different to a British family brewer continually repitching into essentially the same wort year after year.
It's only an undergrad project, but
this suggests that if two strains have co-evolved, then it is possible to maintain what starts as a 50:50 blend over 5 repitches in 2-week ferments - but the proportion is quite sensitive to environmental conditions, and a blend of less-well-adapted strains just falls apart within the first fermentation.
I'll agree there's nothing conclusive, but at the same time I don't think you can dismiss it as just "interesting abstract theory".