The Federal Reserve has offered more than $3 trillion in loans and asset purchases in recent weeks to stop the U.S. financial system from seizing up, but it has not yet directly helped large swaths of the real economy: companies, municipalities and other borrowers with less than perfect credit. That is partly because America’s central bank is not allowed to take much credit risk itself, and loans to lower-rated borrowers have a higher chance of losses. The risk is exacerbated by the spread of coronavirus which have brought economic activity to a screeching halt. To alleviate that constraint, the U.S....