I would like to submit a new KIP to complete the changes made to the KafkaLog4jAppender. Although this does enable to ability to support SASL it does not extend to the use of SASL Authentication CallbackHandlers rendering it incomplete. It's a small change however my understanding is that it will still require a KIP.
https://cwiki.apache.org/confluence/display/KAFKA/KIP-425%3A+Add+some+Log4J+Kafka+Appender+Properties+for+Producing+to+Secured+Brokers
