Aug 24 2016
Of course it would by a breaking change. There is no formal spec of the JSON dump beyond the spec for the individual entities, but we have always said that the dump is a set (an array) of entities. Putting something in there that is not an entity will break consumers.
Aug 23 2016
Right now, the JSON dump format is a sequence of JSON objects. Each of these JSON objects is a Wikidata entity. There is nothing preventing the dump format from having the first JSON object be information about the dump, including version of the dump format, version of wikidata format, time of dump, etc.