Why isn't the other value just a oneof? I get if your holed value is passthru data encoded in some special way that isn't standard asn1 or proto, but at that point it's heavily application-dependent and not really the outer protocol's job to support.
You can do CHOICE in ASN.1, yes, and you can even make it an extensible CHOICE. In that case the tag is the type determinant, and it looks a lot like a typed hole. But! sometimes you want a typed hole where the type determinant is something like a URN, or a URI, or some other type where the value space is a) large, and b) structured so you can avoid needing a registry. And sometimes the protocol you're writing inherently can't have a type registry -- think of an RPC layer where you have headers that provide things like authentication and negotiation of things, session-like things, while the application provides the procedures (the 'P' in RPC) and so you need to identify the application without a registry of oneof tags.
