If the full faith and credit of the United States government ain't worth squat (which is the risk you're describing, essentially), then I'm not sure either what trust we should put in equities or your idea. The problem of redeeming equities and bonds are even greater in the private market! (My guess is that huge pension programs like California's has a staff dedicated to selling/redeeming stuff that plans very carefully.)

The general question is one of competing risks. The pay-as-you-go system involves the risk of the fed's defaulting on bonds and the demographic projections of actuaries. Other systems involves the risk of screwing up not only Social Security but the anonymous nature of stock transactions.