A proposed rule introduced last week could give the Federal Home Loan Banks greater responsibility and flexibility in managing their growing mortgage-purchase programs. But some critics are concerned about a provision that could entail unwanted participation by non-members. Under the current acquired member asset (AMA) regulation, which went into effect July 2000, FHLBanks can invest in mortgages acquired from their members through risk-sharing arrangements. But FHLBank officials have complained that existing AMA regulations have