Linked to https://phabricator.wikimedia.org/T207437, which would probably need to be implemented before or at the same time as this.
The basic idea would be to have an option in the Page Curation toolbar to mark an article as 'Under Review' and then to give some sort of warning at Special:NewPagesFeed that the article is currently 'under review' (set to expire in 5 min in case the reviewer moves on).
Suggested here: https://en.wikipedia.org/wiki/Wikipedia:Page_Curation/Suggested_improvements#44._Review_'on_hold'