Optimizely has this Engagement goal set as a default goal in every experiment. It measures any click on any element. Other than this you have no idea of the industry standard bounce rate and they are quite different in nature. It is unthinkable for me to not measure the bounce rate as a primary KPI for a landing page which Optimizely should be designed for. Engagement is such a vague term. Getting accurate bounce rate during A/B tests especially if you use redirects to other domains or subdomains is such a difficult task and misleading.
Man, just give us the bounce rate.