Finally, I decided to use Configuration class with ConnectionFactoryCustomizer bean depending on spring.rabbitmq.ssl.enabled property:

@Configuration
class RabbitConfig {
    @Bean
    @ConditionalOnProperty(name = ["spring.rabbitmq.ssl.enabled"], havingValue = "true", matchIfMissing = false)
    fun connectionFactoryCustomizer(): ConnectionFactoryCustomizer {
        return ConnectionFactoryCustomizer { 
            it.saslConfig = DefaultSaslConfig.EXTERNAL
        }
    }
}