Press reports in June that retired television personality Ed McMahon faced foreclosure on his Beverly Hills, California, home — for which he had borrowed $4.8 million from Countrywide Financial Corp. — sparked speculation that the credit crunch was spreading into the high-net-worth sector. Indeed, as one form of debt after another has fallen victim to the crisis that has roiled the financial industry over the past year, jumbo mortgages have gotten hard to find, at least from traditional mortgage sources.

It’s paradoxical: The wealthy borrowers who can afford jumbo mortgage loans — those exceeding the limit of $417,000, or in some high-cost locations $729,750, for sale to the now-government-controlled Fannie Mae and Freddie Mac — wouldn’t seem to pose a high default risk, yet capital-strapped mortgage banks have all but closed off that high end of the market. This is so even though...