Support for Confluent Schema Registry?

We are using https://github.com/takebayashi/fluent-plugin-avro to format our messages as Avro, but when sending to our kafka cluster, we need to prefix the avro data with 5 bytes to conform with the Confluent Schema Registry, as described here:

https://docs.confluent.io/current/schema-registry/serializer-formatter.html#wire-format

Is this possible? Is there a buffer or filter plugin that allows us to modify the raw bytes sent to Kafka after it has been formatted but before it is sent?