We don’t provide that exact functionality natively, but I think you could be able to do it by using some sort of external lazy loader that doesn’t insert the SmartVideo block until it is in viewport.
We do lazy loading automatically on videos for speed purposes, but in an autoplay scenario, that is bypassed on our side. That is why something external by be needed. I am not familiar with any plugins off the top of my head that I can recommend, but certainly seems like something that would exist in the vast WordPress ecosystem.