I like this idea! Lately we’ve been trying to pull libraries out of the nursery, and one of the side-effects is that the nursery libs team on GitHub remains crate owners along with the new team (the more the merrier). Solidifying that side effect in a bus-factor team for the wider ecosystem sounds great to me. I’ll just go ahead and blat some of my own disorganized thoughts now 
It’s probably not likely the team will need to act anytime in the near future so it might be good to work out some semantics and capture them in a repository under that organisation you created first?
The big questions for me are:
- what is an abandoned crate?
- what does the team do with abandoned crates?
From that we can answer what does it mean for the bus factor team to share ownership over a crate?
Different crate authors would probably have different opinions and expectations. Finding a balance of proactive and reactive responses to keeping the ecosystem moving forward is probably going to need some more thought. I think a library needs to be in a particular state of maturity before you could proactively shift maintainership if the current maintainer shifts their attention elsewhere temporarily and have it then shift back.
Personally, I’d also be cautious about conflating shared ownership of a collection of crates with shared maintainership of that collection right now. So when faced with an abandoned crate, my preference would be towards facilitating its handover to a new group of maintainers, rather than having the bus factor team take on that maintenance responsibility (handover might involve triaging, working through outstanding PRs, setting up communication channels, but not indefinitely). The reason is that in my experience, the bigger and less cohesive a collection of libraries gets the harder it is to give each library the dedicated, domain-specific maintenance attention it needs.
Anyways, I’m excited about this idea, and would love to get on board and help out too!