While custom firmware is not possible (but it's just about size of the order - call Cisco sales department) the SPA112 still supports zero touch provisioning. It may or may not help you (you didn't disclosed the details of your environment).
But note the SPA112 is declared EoL (The last day to order is June 1, 2020). and it's out of stock already. So your chances are low even for very large orders.
You should consider ATA191-3PW-K9 instead. Moreover it claims support for zero-touch provisioning via TR-069.
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