I am creating a set of diagrams that are used together, and exist as several
separate .vsd files. Each of the files has several pages/tabs, but they all
use the same background.

I am hoping to put the backgrounds in a separate file and point foreground
pages to it, so I don't have to maintain the same background in each of the
many diagram files.

So the question is, can you configure a foreground page to use a background
that exists in an external file?