I called it updatable rather than "writable" or "read-only" because it might perhaps be extended in the future with separate options for "insertable" and "deletable". It could also be extended to give column-level control over updatability, or something like "use_remote_updatability" could be added, but that all feels like 9.4 material.
Yes this is definitely material for 9.4. You should add this patch to the 1st commit fest. I'll add myself as a reviewer.