As outlined at https://www.oracle.com/corporate/acquisitions/dyn/technologies/migrate-your-services/ OCI DNS does not support Dynamic DNS yet.
Regardless, you may try with the method you show above. You should be able to obtain the recordHash of a record with a prior GetZoneRecords.
Thanks for the pointer re GetZoneRecords. Turns out it IS in fact possible to modify IPv4 resolution by obtaining the appropriate JSON from GetZoneRecords, and then modifying the rdata value in the "A" record via a PUT to UpdateZoneRecords. The remaining piece of scheduling checks of current NAT address and updating OCI DNS resolution information as appropriate is trivial to implement.