FWIW, it came down to this:

AccrInt does not exist for this:
myResult = gExcelApp.WorksheetFunction.AccrIntM(myPaymentDate Last,
mySettlementDate, myCouponRate, myParAmount, myBasis)

But this does the job:
myResult = gExcelApp.Run("AccrIntM", myPaymentDateLast,
mySettlementDate,
myCouponRate, myParAmount, myBasis)