Scrum Forum

Do you have reason to believe that either of these constructs exist in Scrum, and are valuable?

What would either offer in terms of an improved accountability for creating Done, integrated increments of work?

Could anybody explain me the differences between a metascrum and an scrum of scrums? 

The difference depends on the variation of how your organization chooses to scale Scrum. All of the Scrum scaling options have some form of this.  I posted a bit more info in this earlier forum topic (https://www.scrum.org/forum/scrum-forum/28789/meta-scrum-teams). 

The Scrum Guide does make any reference to a Scrum of Scrums or a MetaScrum.  These are terms that have developed outside of the Scrum framework.  

For example, the term Scrum of Scrums was mentioned in an article written by Jeff Sutherland in 2001.  Here is my source for that information. 

MetaScrum is a term used in Scrum@Scale.