Having a Cash-Only SCO register would be very desirable, as it could be truly customized with state-of-the-art technology for handling the acceptance and disbursement of paper currency bills. It would essentially meet the robustness standard of a bank-branch ATM. The SCO attendant would be trained to periodically check the cash supply in both the "accept" and "disburse" functions a couple of times during the day. In other words, the combination of a more RELIABLE register combined with scheduled periodic maintenance of the cash-handling supply would make this more dependable and seamless.
A reliable Cash-Only SCO register does entail time usage in handling the paper currency and coins, but cash transactions are exempt from credit-card and debit-card "swipe fees". Those card issuer swipe fees are quite onerous, ranging between 2% and 4% of the transaction, and Target has participated in retail-industry lawsuits against the card networks and card issuers over those costs.
Having separate Payment-Card-Only and Cash-Only SCO registers is a good idea.