First, make sure there really aren't public admin REST API calls exposed for your use case, by reviewing the reference at https://www.keycloak.org/docs-api/latest/rest-api/index.html

While I am not familiar with what public APIs/SPIs Keycloak actually provides, looking at the linked Javadoc, these do not appear public API/SPI. Which means that things can change without warning in the next version. Nevertheless, you should be able to implement a Keycloak extension to achieve what you need, see linked extensions for reference/inspiration: https://www.keycloak.org/extensions