I think this would be a useful addition. A couple of problems:
This change in the comment doesn't make sense to me and doesn't seem to match the code:
- /* If we have COPY [BINARY] <sth>, complete it with "TO" or "FROM" */ + /* If we have COPY|BINARY <sth>, complete it with "TO" or "FROM" */
The list of commands to allow as the "query" inside the parentheses is documented to be: SELECT, VALUES, INSERT, UPDATE or DELETE; and actually TABLE should also work. Your list doesn't include all of those. So please adjust that.