Hi,
I'm pleased to announce a new (or perhaps, revived) extension for MediaWiki, the EtherEditor. It's a way to allow collaborative editing of any wiki page.
I quickly tested, the integration seems clean to me.
A good question would be "How to define best practices to save the revision (e.g. credits to the people having participated to the collaborative edit)?".
A /different /approach is my extension https://www.mediawiki.org/wiki/Extension:EtherpadLite (in SVN) which was developed some months ago. It's target is not to replace the MediaWiki editor, but to embed one or more Etherpads (Etherpad lite) in frames_in_ a rendered MediaWiki page. In that respect it differs from your announced EtherEditor extension.
I prefer the Mark approach as more universal, as it solves the need "Edit together an article", and yours is more "Embed a document". The multi frames screenshots page don't seem very ergonomic.