We have a very similar capability today via the content interface. For relevant cases, usually involving large payloads rather than an individual library, one can define a content interface and share that content across several snaps. One of the reasons we went with this instead of using a digest as suggested is that this approach allows the snap providing the content to also be updated and get serious issues fixed without having to rebuild every single dependent snap. This is an approach we’re putting in place for the gnome libraries, and it looks like kde is also following the same route.
For individual libraries and simpler cases, I wouldn’t worry so much though. The space savings are not worth the complexity cost and risk, even more in a system such as snapd that makes good use of deltas.