If you read the protobuf source, you can see a bunch of places where you can hook in custom type-fetching code, e.g. in the google.protobuf.Any type.

After studying it a bit, I'm certain this is how it's used inside Google (might also be mentioned elsewhere).

All you'd really need to do is to compile all protos into a repository (you can spit out the binary descriptors from protoc), then fetch those and decode in the client.

It'd actually be quite straightforward to set up,