Sure, but many people would opt in, especially if you explained that they would benefit.
Maybe. Maybe not. Before committing to developing such a thing, you'd have to at least do some research and analysis to find out if that's true and how the likely opt in/out ratios would impact the business case. Remember, this is hosting content in a daemon on your machine .. I think that'd a non-starter for a lot of people, despite the upside of shorter shader updates. (I'm not super up on what the US ISP market/landscape is like these days, but are not data caps still a thing on many plans there? I get the sense that hosting off a home line is not only a performance concern but a concern with actual possible financial ramifications.)
It can't be only when the game is open - this is when gamers are most sensitive to their computers doing other work, and the available of such a network would be far more limited.