Isn't this a project for archive.org? Doesn't archive.org hopefully already have all of this content?
You'd hope someone is doing something about it, but it doesn't appear anyone is. I'm pretty sure a large chunk of old feeds are about to disappear for good.. the main problem is that even if archive.org were doing something about it, there is no good proxy of RSS feed popularity: you need the subscription lists to see what to mirror. So they'd be shooting blindfolded.