Within the master pages I've got one A4 landscape page, which should be repeated on several other A3 master pages (portrait) within the same document (so I only would have to make changes once). This may not be a link to an external document!
It looks like this:
|Master A (A4 landscape)||Master B (A3 portrait)||Master C (A3 portrait)|
|Top: content B||Top: Master A|
|Content Master A||Bottom: Master A||Bottom: Content C|
Since CS6 you have the content tools. You can pick content with the one tool, then the conveyer shows up in the middle on the screen. Click on link and on the keep icon, then change to the content placer tool. Place wherever you need it again.
Later, make changes in the original only, updates the instances in the link panel.
Another method could be:
Make the Master A.
Master B make as Child of Master A, extend to the top with the page tool
Master C extend to the Bottom with the page tool.