One of the black swan events is just a moderate to major market correction, say 30% off current stock prices, which would very likely bring Berkshire down from a market cap of $1.1t to, say, $800b. With $300b in cash available, he could set up a Dutch auction and buy back close to 40% of shares, solving the problem of the anchor that the cash represents, and returning Berkshire to a $500b market cap where there are more acquisition targets of the appropriate size. Then he could massively increase the repurchases going forward, in an attempt to shrink and not expand the size of the company.

A few comments here:
1. A 30% downdraft isn't really a "black swan", it's a semi-regular occurrence that happens every decade or two (6 times already since the 50s).
2. If Berkshire drops by 30%, would you think that "everything else" drops by less, the same, or more than Berkshire dropped?
3. I don't think a dutch auction would work for 40% of the shares, heck it probably wouldn't even work for 5% of the shares. That's because many people (more than most other stocks) hold tight to their Berkshire shares and rarely if ever even consider selling them. For most large holders, the shares only ever get sold when they die, and even then, they are often donated and then slowly sold off to fund various good works.
4. Look above at item 2. Most of us believe that Berkshire is of higher quality than the vast majority of other companies. So it stands to reason that if Berkshire drops a lot, the others will drop even more. And when that happens, maybe it would be smarter to purchase other company's stock at a greater discount than purchasing Berkshire at a lower discount.