You could probably do it with JS. You would have to look into it, get some help etc. Plenty of front-end devs out there to collab with.
I agree, it's a common-sense feature. After all, SQS's main thing is "rich experiences" that are easy to create and manage. Playing multiple videos on load or when playhead is clicked makes sense to me.
For now, in 7.1 I'm using full bg video on our new site (sorry, no link yet). The main problem here is that it's YouTube, Vimeo or bust - take your pick. Whereas Squarespace should allow us to upload our clips natively, skipping the nonsense of 3rd part