That would be pushing the limits of how Webflow operates.
It is a design tool that allows designing then publishing individual websites in their own containers, so the extent of what you can do is limited to sharing static elements such as CSS and images between projects.
While it does support CMS items and ecommerce features, like its competitors, the focus is on the design at the page level - Unlike a theme based CMS like Drupal, Wordpress, Concrete, or Kentico.
What you are looking for is not implemented at the Webflow level and you do not have access to the code to modify this yourself. Even if you were loading parts from a site via AJAX calls into other websites, it would take more time to get the hack working than to maintain the individual sites when there is an update.
It seems that the sites share a lot, if you have to update them all for each layout change on the master.
If I may ask, what is the relation between your core website and its dependents?
Is it a localisation issue? or are they all completely different sites? Or is it a requirement per domain name? …