There is no reason why a bank couldn't allow customers to use FedNow to access a revolving credit line.
If I lend out $X dollars but my client says the loan was accessed fraudulently then who pays for this loss? The bank, the payment processor (FedNow/Visa), the customer, or the vendor?
Liability is on the receiving or the originator institution. But in practice, it depends on the contract with the “processor”. Many Pay By Bank “processors” offer a guarantee model to cover these returns. Otherwise, liability is typically on the merchant.
However, Nacha is beginning to iterate on their return codes to better fit the e-commerce use case and clearly define liability.