"It would mean coordinating updates of the digest to all referencing sites though, which might make it practically impossible."
You just put the version number in the name somewhere and make deployments immutable. Otherwise you have everything you need for that already, technically, though you might want to add a fallback to a local clone of the resource or something so it isn't just a failure.
Using anything from sources you don't trust is a bad idea, but even trustworthy partners can be exploited to attack your site as well.
It would mean coordinating updates of the digest to all referencing sites though, which might make it practically impossible.