Here is what I think you are looking for, an explanation of why in the dividend discount model, g cannot exceed r. The excerpt is from the finance textbook, "Essentials of Investments," by Bodie, Kane, and Marcus; McGraw-Hill; 1998; page 360:
"The constant growth DDM (i.e., PV = C / (r - g)) is valid only when g is less than r. If dividends were expected to grow forever at a rate faster than r, the value of the stock would be infinite. If an analyst derives an estimate of g that is greater than r, that growth rate must be unsustainable in the long run. The appropriate valuation model to use in this case is a two-stage DDM."
So you see, g may be greater than r in the short-run -- but not in the long run. As I mentionned before, Buffett plugs in "owner earnings" instead of dividends as "C" in PV = C / (r - g). Incidentally, you'll find a similar paragraph like the above in every finance textbook you open (look up "dividend discount model" in the index).
To reiterate, your point about not being able to use the perpetuity formula when g is greater than r is well-taken, however the case never arises that a company's g will be greater than the discount rate, r, in the long run. A company's g may be greater than r temporarily but eventually it will fall below r. The reason is because of a company's life cycle, wherein a company will experience a growth or "exponential" stage, followed by a slower "maturity" phase. For such companies, a "two-stage" dividend discount model is appropriate, the first stage corresponding to the company's exponential phase, wherin g > r, and the second stage corresponding to its maturity phase, wherein g < r. Buffett himself often talks about Berkshire not being able to grow at 24% forever. He cites Carl Sagan's example of bacteria: though they could double in mass every 20 minutes forever into the future, they don't because the world (and bacterial substrate) is finite. Likewise, any company's positive-NPV projects are finite. Coke will reach its "maturity" phase when every man, woman, and child on Earth drinks 10 Cokes a day (like I do).